Twenty-one Israeli soldiers were killed Monday when they came under attack in the southern Gaza Strip, triggering a blast that collapsed two buildings with soldiers inside them, the military said Tuesday morning.

The buildings were being rigged for demolition by troops when Palestinian gunmen fired an RPG at a tank securing the forces. A second blast then occurred in the buildings, possibly as a result of a second RPG, leading to their collapse.

It was the single deadliest incident since the start of Israel’s ground offensive in the enclave, and raised the military death toll in the operation to 219.

Twenty-one Israeli soldiers were killed Monday when they came under attack in the southern Gaza Strip, triggering a blast that collapsed two buildings with soldiers inside them, the military said Tuesday morning.

The buildings were being rigged for demolition by troops when Palestinian gunmen fired an RPG at a tank securing the forces. A second blast then occurred in the buildings, possibly as a result of a second RPG, leading to their collapse.

It was the single deadliest incident since the start of Israel’s ground offensive in the enclave, and raised the military death toll in the operation to 219.

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At a press conference Tuesday morning, Israel Defense Forces Spokesman Rear Adm. Daniel Hagari said that troops were operating in an area around 600 meters from the border, across from the southern Israeli community of Kissufim. They were destroying structures and Hamas sites as part of the army’s efforts to establish a buffer zone to allow residents of Israeli border communities to return to their homes.