Islamist rebels have claimed responsibility for a massive explosion in Syria’s contested second city of Aleppo Thursday morning.

The attack which demolished the Carlton Hotel, which was reportedly being used as a military headquarters, was claimed by the Islamic Front via Twitter.

The group claimed that it had targeted “what remained of the Carlton Hotel in #Aleppo, killing more than 50 of the Assad mercenaries”.

Those casualty figures could not be independently verified, although the London-based Syrian Observatory for Human Rights confirmed the attack had been carried out by the Islamic Front.

The Observatory claimed rebels had detonated a large quantity of explosives in a tunnel they had dug underneath the hotel. It would mark the second such attack in as many days; around 30 regime fighters were reported killed yesterday in a similar “tunnel bomb” attack in the northern Idlib Province. That attack was also claimed by the Islamic Front.

The new wave of attacks come as rebels continue to lose ground to forces loyal to Syrian President Bashar al-Assad, ahead of presidential election in early June – a poll which has been dismissed by a farce by western states.

On Wednesday the rebel “capital” of Homs, which has been at the center of the three-year uprising against Assad’s rule, finally fell to pro-government forces after a prolonged siege. A ceasefire agreed between the sides allowed rebel fighters to evacuate safely in exchange for the release of dozens of captured Hezbollah fighters and Iranian agents.

The latest style of attacks may indicate a new phase in the rebellion, as rebel brigades adapt to the loss of territory by adopting insurgency tactics.

A report in The Times last year suggested the use of tunnels specifically by rebels may have been learned from Hamas, which has been treading a fine line between the Assad regime, backed by Hamas’s one-time major sponsors Iran, and its fellow Sunni Islamists in the Syrian rebel movement.