Twenty-one soldiers were killed when rocket-propelled grenade (RPG) fire hit a tank near two buildings where they were carrying out an operation, military spokesman Daniel Hagari said in a televised statement.

He said the attack occurred at around 4 pm (1400 GMT) on Monday and all those killed were reservists.

The RPG was fired at a tank that was protecting the troops when an explosion erupted in two nearby two-storey buildings, Hagari said.

"The buildings collapsed as a result of this explosion, while most of the force was inside and near it," he said.

The buildings exploded as troops had planted explosives in them after the two structures had been identified as "terrorist infrastructure" in the area, Hagari said.

A medical evacuation team was deployed but it was a "complicated operation, which took place until the last hours," Hagari said, indicating the difficulty in extracting bodies buried under the rubble.

"The dedicated reservists, who stood up for the flag, sacrificed what was dearest to them for the security of the state of Israel so that we can all live here in complete safety," he said.

The army had previously announced the deaths of three other soldiers on Monday, taking the day's overall toll to 24 -- the deadliest since the ground offensive started in Gaza.

Israel began a sweeping offensive against Hamas militants in Gaza after the Palestinian group attacked southern Israel on October 7.

That attack resulted in the deaths of around 1,140 people, mostly civilians, according to an AFP tally based on Israeli official figures.

Israel's blistering retaliatory offensive against Hamas in Gaza has killed 25,295 people, most of them women, children and adolescents, according to the health ministry of the Hamas-run territory.