A large explosion in the northern Syrian city of Aleppo has destroyed a hotel and several other buildings, state media and activists report.
Rebel fighters are believed to have detonated a bomb placed in a tunnel beneath the Carlton Citadel Hotel, near the city's medieval citadel and souk.
The UK-based Syrian Observatory for Human Rights said government troops had been based there and at least 14 died.
Both sides have been trying to end a long-standing stalemate in the city.
In recent weeks, rebels have been trying to advance on areas where government forces are entrenched, while rebel-held areas of Aleppo have come under fierce aerial bombardment since mid-December.
The state news agency, Sana, reported that "terrorists" had blown up tunnels they had dug underneath archaeological sites in the Old City.
Preliminary reports said the hotel had suffered "huge damage", it added, without saying if there had been any casualties.
The Carlton Citadel is situated inside a 150-year-old building that faces the entrance of the 13th-Century citadel, which along with the rest of the Old City is a Unesco World Heritage Site.
The Syrian Observatory for Human Rights and the opposition Shaam News Network (SNN) said it was being used by government forces.
The remote detonation of a large quantity of explosives placed in the tunnel by the Islamic Front had destroyed the hotel and caused the collapse of several nearby buildings, the Observatory said.
A number of security forces personnel and militiamen loyal to President Bashar al-Assad were believed to have been killed, it added.
Photographs and video published online purported to show the moment of the blast, with a cloud of smoke rising from the scene.
A statement from the Islamic Front said its fighters had "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 BBC's Paul Wood in Beirut says the attack shows how fiercely contested the city of Aleppo remains.
The front lines have moved little in more than two years of fighting, though it seems the rebels have made a few incremental gains in recent months.
Our correspondent says that is significant. Government forces are doing well in Damascus and have just assumed control of the central city of Homs, but in the north of the country they seem to be under serious pressure and the military stalemate seems as durable as ever, he adds.