Using an underground bomb, Syria’s rebels struck a key regime position in northwest Aleppo on Wednesday, reportedly causing dozens of casualties.

The explosion, which registered as a 3.0-magnitude earthquake on monitors in Turkey, damaged the Air Force Intelligence building in the al-Zahraa district. In addition to being a military headquarters, the fortified complex has been a key “watch” position for regime forces in an area under attack by the opposition since 2012.

The attack, including the tunnel for the bomb, had been planned for months by a group of rebel factions, including the Islamic Front and Jabhat al-Nusra. It succeeded despite extensive regime measures put in after a series of detonations in Aleppo in 2013 and 2014, including well-trained and well-armed Syrian troops in a network of tunnels and underground facilities.

Rebel sources claim — contrary to assertions by pro-Assad outlets — that the building was occupied and had not been evacuated when the explosion occurred, leading to a high death toll. The explosion was followed by a ground assault, causing further casualties.

One source explained the significance of the attack for rebel advantage in further assaults on the regime in northwest Aleppo:

This was the last fortress hindering the liberation of nearby neighborhoods. Rebels can now tighten the frontlines to put pressure on remaining regime-held areas and probably advance into these now that they don’t need to fear attacks from the fortress.

State media ignored the bombing and claimed, “The army foiled an attack by terrorist organizations on al-Maliya square and the air intelligence branch, killing a number of terrorists.” The army reassured “that the situation is under full control“ and insisted only six troops died.

Pro-Assad accounts on social media claimed that the building had little military significance.