JERUSALEM — ISRAEL increased airstrikes on the Gaza Strip and sealed it off from food, fuel and other supplies on Monday in retaliation for a bloody incursion by Hamas militants, as the war’s death toll rose to nearly 1,600 on both sides.

Hamas also escalated the conflict, pledging to kill captured Israelis if attacks targeted civilians without warnings.

In the war’s third day, Israel was still finding bodies from Hamas’ stunning weekend attack into southern Israeli towns. Rescue workers found 100 bodies in the tiny farming community of Be’eri — around 10% of its population — after a long hostage standoff with gunmen. In Gaza, tens of thousands fled their homes as relentless airstrikes leveled buildings.

The Israeli military said it had largely gained control in the south after the attack caught its vaunted military and intelligence apparatus completely off guard and led to fierce battles in its streets for the first time in decades.

Hamas and other militants in Gaza say they are holding more than 130 soldiers and civilians snatched from inside Israel.

Israeli tanks and drones were deployed to guard breaches in the Gaza border fence to prevent new incursions. Thousands of Israelis were evacuated from more than a dozen towns near Gaza, and the military summoned 300 000 reservists — a massive mobilisation in a short time.

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.

“We have only started striking Hamas,” Israeli Prime Minister Benjamin Netanyahu said in a nationally televised address. “What we will do to our enemies in the coming days will reverberate with them for generations.”

As the Israeli military brought additional forces near the border, a major question was whether it will launch a ground assault into the tiny Mediterranean coastal territory. The last ground assault was in 2014.

The Israeli military said more than 900 people already have been killed in Israel. In Gaza, more than 680 people have been killed, according to authorities there; Israel says hundreds of Hamas fighters are among them. Thousands have been wounded on both sides.

In response to Israel’s aerial attacks, the spokesman of Hamas’ armed wing, Abu Obeida, said Monday night that the group will kill one Israeli civilian captive any time Israel targets civilians in their homes in Gaza “without prior warning.”

Israeli Foreign Minister Eli Cohen warned Hamas against harming any of the hostages, saying, “This war crime will not be forgiven.” Netanyahu appointed a former military commander to manage the hostage and missing persons crisis.