The Israeli army said on Tuesday that 21 soldiers were killed in the central Gaza Strip, marking the deadliest single incident for the troops in the more than three-month-old war against Hamas militants.

Army spokesman Daniel Hagari announced the deaths following the lifting of a news blackout that had barred reporting on the case.

Israeli media reported that the soldiers were killed when two buildings collapsed after Palestinians fired anti-tank missiles at the troops.

Fierce fighting and Israeli attacks, in which many Palestinians were also killed, had recently intensified in the area around the southern city f Khan Younis in particular. Israel suspects the leadership of the Islamist Hamas and Israeli hostages in the tunnel network.

According to military figures, 220 soldiers have been killed since the start of the Israeli ground offensive in the Gaza Strip at the end of October. More than 1,200 others have been injured.

The war was triggered by the worst massacre in Israel’s history, carried out by terrorists from the Palestinian group Hamas and allied extremists. About 1,200 people were killed in the attack on southern Israel, including hundreds of soldiers.