A hotel in the Syrian city of Aleppo used by government troops as a headquarters was destroyed in a huge explosion this morning when opposition groups detonated explosives in tunnels underneath the building.

Video footage posted online purports to show the moment of the explosion.

Syrian state television identified the target as the Carlton hotel in the old part of the city. The state-run news agency SANA said that “terrorists” — which is how the station labels anyone fighting against government forces — had blown up tunnels in Aleppo's old city.

The UK-based Syrian Observatory for Human Rights, which uses a network of activists on the ground, also reported the attack, and said that the hotel was being used as a base for Syrian President Bashar al-Assad's forces in the area. The explosion, it added, leveled the hotel and caused the collapse of surrounding buildings. It said there had been casualties among government troops resulting from the explosion and that both sides had lost men in subsequent violent clashes.

Casualty figures are impossible to verify, but the Aleppo-based opposition activist Sham News Network said “a large number “of government soldiers had been killed or injured. SANA did not mention the number of dead or wounded.

The Islamic Front grouping claimed responsibility for the attack in a statement posted on it’s official Twitter account, saying: “Our fighters blow up what remained of the Carlton Hotel in #Aleppo, killing more than 50 of the Assad mercenaries.” It is unclear how it arrived at the number of dead.

The hotel is situated in Aleppo's old quarter, a UNESCO World Heritage Site. Some of the district may also have been damaged in the blast. SANA said the attack was on “Aleppo old city antiquities" and "left huge damage to the historical site according to primary estimations.”

An activist post on Facebook reported that a “huge explosion” caused its destruction of the hotel and “a number of nearby monuments.”

The timing of the attack is significant. Assad's troops have made significant recent gains in the capital of Damascus and yesterday took control of Homs, Syria's third largest city. However, the stalemate between opposition and government forces in Aleppo and the rest of the north of the country looks set to continue.

The Islamic Front also claimed responsibility for a similar attack two days ago at a government checkpoint in Idlib province. The group was formed last year via the merger of several Islamist militant groups fighting against Assad’s forces and aims to establish an Islamic state in Syria. However it has also clashed with the extremist Islamic State of Iraq and the Syria (ISIS).