An attack on Israel’s forces in the Gaza Strip on Monday left 21 soldiers dead, its military said Tuesday. The attack was the deadliest for Israeli troops since the Hamas-led massacre on Oct. 7 that triggered the war.

According to the Israeli military, reservists were preparing explosives to demolish two buildings in central Gaza when a militant fired a rocket-propelled grenade at a tank nearby.

The blast from the rocket outside the buildings triggered the explosives inside them, causing both two-story buildings to collapse on the soldiers.

Prime Minister Benjamin Netanyahu mourned the loss of the soldiers, saying Monday was “one of the hardest days” since the war began.

At least 217 Israeli soldiers have been killed since the ground offensive began in late October, including three in a separate event on Monday, according to the military.

Monday’s attack could become a major setback to renewed calls for a cease-fire as Netanyahu vowed the IDF will continue its war with Hamas until “absolute victory.”

“In the name of our heroes, and for our own lives, we will not stop fighting until absolute victory,” he wrote on the social media website X, formerly Twitter.

Hours after Israel announced the attack, its military said ground forces had encircled the southern city of Khan Younis, where Israel claims Hamas’ leaders are in hiding.

Heavy fighting has persisted in and around the city – Gaza’s second largest – in recent days. The fighting continues amid efforts for a second cease-fire.