The military wing of the militant Hamas group says it will kill one Israeli civilian captive any time Israel targets civilians in their homes in Gaza “without prior warning”.
The threat came as the death toll in Israel rose to 900 on Monday evening, according to Israeli media reports.
The Health Ministry in the Gaza Strip said more than 680 people have been killed in Israel’s retaliatory strikes following the unprecedented Hamas attack on Saturday, with more than 3,700 people have been wounded.
Israeli rescue service Zaka said more than 100 bodies have been recovered from a small farming community that was the scene of a hostage standoff during Hamas’ attack.
The figure is part of the total 900 reportedly killed in Hamas’ multi-pronged attack. Beeri, a kibbutz, had a population of about 1,000 people before the attack.
“We have decided to put an end to this and as of now and we declare that any targeting of our people in their homes without prior warning will be regrettably faced with the execution of one the hostages of civilians we are holding,” he said.
In a video statement on Monday, Israel’s foreign minister warned Hamas against harming any of the hostages who were taken from Israel and are being held in Gaza.
Eli Cohen said Israel was committed to bringing the hostages home “in the spirit of mutual responsibility”.
“We demand Hamas not to harm any of the hostages,” Mr Cohen said.
“This war crime will not be forgiven,” he added.
Israel’s military ordered a complete siege on the Gaza Strip on Monday, halting deliveries of food, water, fuel and supplies to its 2.3 million people as it pounded the Hamas-ruled territory with waves of airstrikes in retaliation for the militants’ bloody weekend incursion.
Israel’s military and intelligence apparatus was caught completely off guard by Hamas, resulting in heavy battles in its streets for the first time in decades.
Israeli tanks and drones were deployed to guard breaches in the Gaza border fence to prevent new incursions.
The moves, along with Israel’s formal declaration of war on Sunday, pointed to Israel increasingly shifting to the offensive against Hamas, threatening greater destruction in the densely populated, impoverished Gaza Strip.
Around 900 people, including 73 soldiers, have been killed in Israel, and more than 680 people have been killed in Gaza, according to authorities on each side.
Israel says hundreds of Hamas fighters have been killed. Thousands have been wounded on both sides.
Hamas and other militants in Gaza say they are holding more than 130 soldiers and civilians snatched from inside Israel.
Earlier on Monday, Mr Biden convened a meeting with top national security aides, including secretary of state Antony Blinken, national security adviser Jake Sullivan, principal deputy national security adviser Jon Finer, homeland security adviser Liz Sherwood-Randall and White House chief of staff Jeff Zients.
During that meeting, Mr Biden urged continued co-ordination with Israel and other regional partners, the White House said.
Mr Biden has spoken at least twice to Israeli Prime Minister Benjamin Netanyahu and the White House says his top national security aides have been in regular contact with their counterparts in the region since the Hamas attack on Saturday.