The Ukrainian President, Petro Poroshenko, ordered his troops in the east of the country to cease fire on Friday night, even as a surge in separatist violence and accusations of deeper Russian involvement in the conflict provoked another round of US sanctions.
A spokesman for the interior ministry said the ceasefire would last for a week and that Ukrainian troops would return fire only if fired upon.
Mr Poroshenko, who was elected last month, has vowed to end the armed pro-Russian uprising which has spread through key eastern industrial cities, sending Ukraine to the brink of a break-up and pummelling relations between Russia and the West.
US and European leaders claim that fighters and weapons are flooding over the border from Russia as Moscow seeks to stoke the instability to regain influence in its former Soviet territory. They have given the Kremlin until the end of the month to clamp down or face tougher economic sanctions.
On Friday the White House said it was adding seven more officials from Ukraine and Russia to a list of dozens who have had their assets frozen and are banned from travelling to the US, and pointed to reports of more military hardware coming over the border.
In pictures: The Ukraine crisis
In pictures: The Ukraine crisis
1/39 Ukraine crisis
Ukrainian soldiers from the 'Azov' battalion guard their base in Mariupol, Donetsk
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Children were transferred from orphanages in Donetsk and Makeyevka to escape the fighting between the Ukrainian army and pro-Russian separatists
3/39 Ukraine crisis
Ukrainian servicemen captured by pro-Russian separatists sit on the ground as they are assigned to clean a street in Snizhne in the Donetsk region
4/39 Ukraine crisis
A view inside the Youth Culture Centre destroyed by pro-Russian separatists in Donetsk
5/39 Ukraine crisis
A woman holds a portrait of her dead son as she speaks during a rally in front of Ukrainian President Petro Poroshenko's office in Kiev
6/39 Ukraine crisis
A boy ascends the stairs of a bomb shelter after the shelling in the Petrovskiy district in Donetsk
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A medic looks at thirty coffins prepared for the funerals of pro-Russian rebels killed during heavy fighting at Donetsk airport, outside a Donetsk morgue
8/39 Ukraine crisis
Relatives mourn near the coffin of Mark Zverev, a taxi driver shot dead during clashes at the Donetsk airport between Ukrainian troops and the pro-Russian rebels, during his funeral in the village of Grabari on the outskirts of Donetsk
9/39 Ukraine crisis
Medical workers carry a wounded pro-Russian militant on a stretcher after armed clashes occured between pro-Russian gunmen and Ukrainian troops in Slavyansk
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A member of a newly-formed pro-Russian armed group called the Russian Orthodox Army mans a barricade near Donetsk airport
11/39 Ukraine crisis
Black smoke rises from a shot down Ukrainian Army helicopter outside Slovyansk
12/39 Ukraine crisis
A woman is overcome with emotion as she looks at blood stains and damage around a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
13/39 Ukraine crisis
A local man looks at damage near a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
14/39 Ukraine crisis
A bloodstained icon of Jesus is seen among blood soaked shattered glass atop a wrecked truck near the Donetsk airport
15/39 Ukraine crisis
A family member attends a funeral for Olga Prokhorenko (60) who was killed by shrapnel after Ukrainian government forces shelled their location, during the funeral in Slovyansk
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A woman reacts after seeing the lifeless body of a man killed by shrapnel following a shelling from Ukrainian government forces in Slovyansk
17/39 Ukraine crisis
Pro-Russian militants take position on the roof of the international airport of the eastern Ukrainian city of Donetsk
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A woman embraces a pro-Russian separatist from the "East" battalion during a rally in the eastern city of Donetsk
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A Ukrainian helicopter Mi-24 gunship fires its cannons against rebels at the main terminal building of Donetsk international airport
20/39 Ukraine crisis
A pro-Russian gunman changes his position near the airport, outside Donetsk
21/39 Ukraine crisis
A pro-Russian gunman aims his weapon near the airport, outside Donetsk. Ukraine's military launched air strikes against separatists who had taken over the airport in the eastern capital of Donetsk in what appeared to be the most visible operation of the Ukrainian troops since they started a crackdown on insurgents
22/39 Ukraine crisis
Pro-Russians ride on a truck in Donetsk. A convoy of an armored personnel carrier and seven trucks carrying several hundred heavily armed men drove through central Donetsk and gunmen got out of the trucks, stood to attention and gave shots in the air in jubilation as a crowd of several thousand supporters cheered them and chanted: "Heroes!"
23/39 Ukraine crisis
An elderly woman leans against the chest of a pro-Russian gunman in Lenin square in Donetsk
24/39 Ukraine crisis
Pro-Russian militants guard a psychiatric hospital after shell explosions during the fighting between pro-Russian militants and the Ukrainian army, in Semyonovka village, outside Slavyansk
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Ukrainian troops stand guard at a checkpoint on the road near the eastern city of Izum, Donetsk
26/39 Ukraine crisis
Members of the Democratic Alliance party take part in a performance in front of the French embassy in Kiev. Protesters set up a pool with the blood of animals and models of warships in a protest against French plans for the sale of two Mistral helicopter carriers to the Russian Navy
27/39 Ukraine crisis
A man carries a portrait of pro-Russian activist Vadim Hudich, who was killed in a shooting incident at the head of a funeral procession in the eastern Ukranian town of Krasnoarmeisk
28/39 Ukraine crisis
A voter is seen inside a voting booth at a polling station during the referendum on the status of Donetsk region in the eastern Ukrainian city of Donetsk
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Members of a local election commission sort ballots as they start counting votes of referendum on the status of Luhansk region in Luhansk
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Members of a local election commission empty a ballot box as they start counting votes of the referendum on the status of Donetsk region in Donetsk
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Members of a local election commission empty ballot boxes as they start counting votes after a referendum, at a polling station in Lugansk
32/39 Ukraine crisis
Local residents watch as others give first aid to a man who was shot in the leg by Ukranian militia in the village of Krasnoarmisk. Eyewitness said that Ukranian militia tried to stop the referendum voters briefly taking the City Hall of Krasnoarmisk, where unarmed pro-Russian supporters were gathering. Reportedly the Ukranian militia came out out the building and started shooting at the people, killing at least one man and leaving at least two others injured
33/39 Ukraine crisis
Members of a local election commission count votes after a referendum organized by the so-called Donetsk People's Republic members at a polling station in Donetsk
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Voters visit a polling station to take part in the referendum on the status of Donetsk
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An Armoured Personnel Carrier (APC) with a Russian flag drives through the center of Slaviansk during the day of referendum organized by the so-called Donetsk People's Republic members in Slavyansk
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A statue of Lenin is placed in front of a pro-Russian barricade on the outskirts of the eastern Ukrainian town of Slavyansk
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A feamle veteran (C) sings during 'Victory Day' celebrations in Donetsk
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People stand near the burning Mariupol police station
39/39 Ukraine crisis
Ukrainian soldiers stand guard beside an armoured personnel carrier at a checkpoint in Mariupol. Ukrainian forces seized the rebel-held city hall in the eastern port city of Mariupol, driving out pro-Russian activists, then withdrew, making no attempt to hold onto the building
“We have information that additional tanks departed from a deployment site in south-west Russia,” a US official said, adding that talks with the EU about moving to the next level of sanctions – debilitating measures targeting specific sectors of Russia’s economy – had intensified.
EU foreign ministers will meet on Monday to discuss new sanctions. Mr Poroshenko is also due to update the bloc on his 14-step peace plan, which went into action on Friday after the Defence Minister told parliament the border with Russia had been sealed. The interior ministry said of the ceasefire: “This is being done so that terrorists can put down their arms; those who do not do this will be eliminated.”
Nevertheless, fierce fighting continued inside Ukraine, with the Associated Press agency reporting that at least seven troops had been killed and 30 injured in the Donetsk region, where separatists declared an independent republic shortly after taking over public buildings in April.
Vladyslav Seleznyov, a Ukrainian army spokesman, claimed about 300 separatists were killed in Friday's fighting. The figures could not be independently verified.
The separatist movement counts anti-government Ukrainians, ethnic Russians, ex-soldiers and a smattering of opportunists among its fighters, and their varying goals and allegiances have complicated efforts to find a political solution to the crisis.
Under Mr Poroshenko’s peace plan, the sealing of the border and the declaration of a ceasefire will be followed by the establishment of a buffer zone along the frontier. There will then be an amnesty for rebels who lay down their weapons and the offer of safe passage to Russian fighters who want to return home.
Mr Poroshenko has also discussed the blueprint with his Russian counterpart Vladimir Putin. Many of the proposals, such as a decentralisation of political powers and greater equality for Russian speakers, are long-standing demands from the Kremlin. Much hinges on whether Moscow finds the terms of the blueprint acceptable and uses its sway with the rebels, who have so far rejected the proposals.
Since its annexation of the Crimean peninsula in February, Russia has claimed no further interest in Ukrainian territory, and earlier this month pulled some of its troops back from the border. On Thursday, the Nato secretary-general, Anders Fogh Rasmussen, said there was evidence that Russian soldiers were regrouping near the frontier. Mr Putin’s foreign policy adviser denied these claims and said only border guards had been deployed.