Russia has proposed a 1 March ceasefire in Syria, US officials say, but Washington believes Moscow is giving itself and the Syrian government three weeks to try to crush moderate rebel groups.
The United States has countered with demands for the fighting to stop immediately, the officials said. Peace talks are supposed to resume by 25 February.
The talk of new ceasefire plans comes as the US, Russia and more than a dozen other countries meet in Munich to try to halt five years of civil war in the Arab country. The conflict has killed more than a quarter-million people, created Europe's biggest refugee crisis since World War II and allowed Isis to carve out its own territory across parts of Syria and neighboring Iraq.
Russia says it is supporting Syrian President Bashar Assad's government as part of a counterterrorism campaign. But the West says the majority of its strikes are targeting moderate groups that are opposed to Assad and Isis.
The most recent Russian-backed offensive, near Aleppo, prompted opposition groups to walk out of peace talks last month in Geneva, while forcing tens of thousands of civilians to flee toward the Turkish border.
The US officials weren't authorized to speak publicly about private diplomatic discussions in the run-up to the Munich conference and demanded anonymity. One said the US can't accept Russia's offer because opposition forces could suffer irreversible losses in northern and southern Syria before the ceasefire ever takes hold.
The officials said the US counterproposal is simple: A ceasefire that is effective immediately and is accompanied by full humanitarian access to Syria's besieged civilian centers.
In pictures: Syria conflict
In pictures: Syria conflict
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Syrians carry children amid debris following a air strike by government forces in the northern city of Aleppo
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A Syrian man carries a girl on a street covered with dust following a air strike by government forces in the northern city of Aleppo
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Syrians react as they stand amid debris following a air strike by government forces in the northern city of Aleppo
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A Syrian man carries a girl amid debris following a air strike by government forces in the northern city of Aleppo
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An injured Syrian man walks out from the rubble of a destroyed building following a air strike by government forces in the northern city of Aleppo
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A Syrian woman makes her way through debris following a air strike by government forces in the northern city of Aleppo
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People stand on the rubble of collapsed buildings at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad, in the Al-Fardous neighbourhood of Aleppo
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Syrian residents stand amid the rubble of destroyed buildings
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A Syrian resident grasps a mattress amid rubble in the al-Firdous neighborhood of the northern city of Aleppo
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A bullet-riddled parking sign stands amid debris in a deserted street leading into the old city of Homs
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A general view shows abandoned buildings on a deserted square in the old city of Homs after Syrian government forces regained control of rebel-controlled areas
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A mosque is pictured through shattered glass in the old city of Homs, as rebel fighters withdrew from the city centre in line with a negotiated withdrawal deal with the government after having held out under tight siege for nearly two years
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Buses carrying Free Syrian Army fighters leaving Homs. Exhausted and worn out from a year-long siege, hundreds of Syrian rebels left their last remaining bastions in the heart of the central city of Homs under a cease-fire deal with government forces. The exit of some 1,200 fighters and civilians will mark a de facto end of the rebellion in the battered city, which was one of the first places to rise up against President Bashar Assad's rule, earning it the nickname of "capital of the revolution"
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Syrian government forces hold up a portrait of President Bashar al-Assad (L) while others raise the national flag on top of a pole in the old city of Homs
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Forces loyal to Syria's President Bashar al-Assad run through Aleppo's Bustan al-Qasr crossing after their release by rebels. They were freed as part of a larger deal which saw the last remaining Syrian rebels in central Homs city evacuate their positions and free captives in several locations in northern Syria
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A Syrian woman and two children walk past heavily damaged buildings in the northern city of Aleppo
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A man carries a wounded girl following a reported bombardment with explosive-packed "barrel bombs" by Syrian government forces in the al-Mowasalat neighborhood of the northern Syrian city of Aleppo
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A wounded man sits as he is treated at a makeshift hospital following a reported bombardment with explosive-packed "barrel bombs" by Syrian government forces in the al-Sakhour district of the northern city of Aleppo
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Debris rises in what Free Syrian Army fighters and Islamic rebels said was an operation to strike Al-Sahaba checkpoint, which is considered a gateway to Al-Dayf valley, and remove forces loyal to Syria's President Bashar Al-Assad in Maarat Al-Nouman, Idlib province
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Men try to put out fire at a site hit by what activists said was an air strike by forces loyal to Syria's President Bashar al-Assad in the town of Azaz, north of Aleppo, near the border with Turkey
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Civil Defence members try to put out fire
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Survivors react at a site hit by what activists said was an air strike by forces loyal to Syria's President Bashar al-Assad in the town of Azaz, north of Aleppo, near the border with Turkey
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Residents queue as they wait to receive food aid distributed by the UNRWA at the Palestinian refugee camp of Yarmouk, south of Damascus
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Belongings of Syrian rebels inside a chapel at Crac des Chevaliers, the world's best preserved medieval Crusader castle in Syria. The village was destroyed in fighting between the government and rebel forces while the castle, listed as a UNESCO world heritage site, also has been damaged over the past two years
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Hosen Sabah, a 16-year-old student is comforted by his mother at a hospital in Damascus. Nosen was wounded by a mortar outside his school, while 14 other students were killed and over 80 wounded
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A Free Syrian Army fighter works on a locally made launcher before firing it towards forces loyal to Syria's President Bashar Al-Assad in Mork town
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Syrian policemen and citizens inspecting the site of a car bomb at the entrance of Moadhamiyet al-Sham neighborhood in rural Damascus. According to Syria's Arab News Agency (SANA), a car bomb explosion has gone off in the countryside of Damascus and initial information say there are casualties, where a car rigged with explosions was remotely detonated at the entrance of Moadhamiyet al-Sham neighborhood in rural Damascus during engineering units it was trying to dismantled it
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Opposition fighters carrying a rocket launcher during clashes against government forces in the Sheikh Lutfi area, west of the airport in the northern Syrian city of Aleppo
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A Syrian man helps a woman to make her way through debris following reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo
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A Syrian man reacts as he carries the body of injured boy following reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo. According to the Syrian Observatory for Human Rights, at least 33 civilians were killed in the attack
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Syrian rescue workers carry the body of a woman following reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo
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Syrians gather at the site of reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo
Secretary of State John Kerry, who arrived in Germany on Wednesday, had talks planned late in the evening with UN peace envoy Staffan de Mistura and Adel al-Jubeir, the foreign minister of Saudi Arabia, a key backer of Syria's rebel groups.
The Obama administration has been trying for months to clinch a ceasefire and pave the way for a transition government in Syria that would allow parties to the conflict to concentrate on defeating the threat posed by Isis and the al-Qaeda-linked Nusra Front.
But after having long demanded Assad's ouster, the shift in the US focus to combating terrorism has resulted in a confusing mix of priorities and a layered strategy in Syria that few understand, and even fewer see working. Beyond Russia, the administration has often struggled to keep its own allies such as Turkey and Saudi Arabia in line.
“We will approach this meeting in Munich with great hopes that this will be a telling moment,” Mr Kerry said on Tuesday in Washington. His peace push coincides with Defence Secretary Ash Carter's attendance at a gathering in Brussels to hash out military options with Nato partners.
Brett McGurk, the Obama administration's point-man for defeating Isis, said Russia's Aleppo offensive was having the perverse effect of helping the extremists by drawing local fighters away from the battle against IS and to the war against Syria's government.
“What Russia's doing is directly enabling Isil,” McGurk told the House Foreign Affairs Committee in Washington.
But the panel's top Democrat echoed some of the frustration of his Republican colleagues with the larger US strategy.
“It seems as if we're only halfheartedly going after Isis, and halfheartedly helping the (rebel) Free Syria Army and others on the ground,” said the Democratic Representative for New York, Eliot Engel. He urged a “robust campaign, not a tentative one, not one that seems like we're dragging ourselves in ... to destroy Isis and get rid of Assad.”
Mr Kerry emphasized on Tuesday that US officials “are not blind to what is happening.” He said the Aleppo battle makes it “much more difficult to be able to come to the table and to be able to have a serious conversation.”
But the US has staked its hopes for an end to the five-year civil war in Syria on the peace talks and Assad's eventual departure, saying the American public has no appetite for a military solution.
To that end, Washington has tempered its calls dating back to August 2011 for Assad to immediately leave power. And to get Russia on board, it now won't even say that Assad should be barred from running for re-election if and when a new Syrian constitution is drafted.
The ambiguity has emboldened Assad's supporters, Russia and Iran, while upsetting American allies in the Middle East, who are frustrated by a process that appears to lock the Syrian leader in place well into 2017 - and perhaps beyond.Reuse content