Syrian rebels have claimed responsibility for a huge explosion which destroyed a hotel and several other buildings in the northern city of Aleppo, activists and state media have reported.
Rebel fighters are believed to have levelled the Carlton Citadel Hotel by detonating a bomb in a tunnel beneath the building.
There were no confirmed details of casualties though state media reported there had been a number of deaths.
The bomb was detonated near to a contested neighborhood in the old part of Aleppo and will be seen as a significant blow to President Bashar Assad's government in the north as his troops prepare to regain control of the central city of Homs following last week's cease-fire agreement.
The attack came as both sides worked to end a long-standing stalemate in Aleppo, Syria's largest city and former commercial hub. The city has been carved up into opposition - and government-held areas since the rebels launched an offensive there in mid-2012, capturing territory along Syria's northern border with Turkey.
In recent months, government aircraft has bombed rebel-held areas of the city and the opposition fighters have hit back, firing mortars into government-held areas. The rebels have also detonated car bombs in residential areas, killing dozens of people.
The hotel attack targeted a government-held area on the edge of a contested neighbourhood in Aleppo. Reports say terrorists - a term the government uses for rebels - dug up tunnels under Aleppo's ancient district and detonated explosives there.
A local activist group called the Sham News Network also reported the blast, saying that Assad's troops were based in the hotel.
The Islamic Front rebel group claimed responsibility for the blast. A statement posted on its official Twitter account on Thursday said that its "fighters this morning levelled the Carlton Hotel barracks in Old Aleppo and a number of buildings near it, killing 50 soldiers". It did not say how it knew how many soldiers died.
The Islamic Front is an alliance of several Islamic groups fighting to topple Assad. Many of its fighters have joined the Front after breaking away from the Western-backed Syrian Free Army last year.
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
Another activist group, the Britain-based Syrian Observatory for Human Rights, said Islamic Front fighters planted a huge amount of explosives in a tunnel they dug below the hotel and detonated it remotely.
It said the hotel was completely destroyed in the blast and that there were casualties among the troops.