Israel has ordered a “complete siege” of the Gaza Strip, cutting off food, fuel, electricity and water supplies while warning of a major troop offensive “within 48 hours” in retaliation for Hamas’ deadly incursion.

As revenge rockets rained down on Gaza, Prime Minister Benjamin Netanyahu vowed to defeat Hamas with “enormous force”, declaring “we are going to change the Middle East”.

“What Hamas will experience will be difficult and terrible,” he said.

Israel is ramping up its military response three days after declaring “war” when Hamas fighters infiltrated the border, leaving more than 700 Israelis dead.

Around 100,000 Israeli troops have massed on the border — the biggest mobilisation in Israeli history — as the army said it would soon go on the offensive.

Chilling hostage threat

As Israeli strikes flattened Palestinian buildings, Hamas issued its own threat that it would execute hostages on live broadcast if civilians were targeted.

More than 123,000 Palestinians living in the densely packed and hemmed-in enclave have reportedly been displaced over the past three days and 600 have died.

“We declare that we will respond to any targeting of our people who are safe in their homes without warning, with the execution of our civilian hostages, and we will broadcast it with audio and video,” said Hamas military wing spokesman Abu Obaida.

Israel’s Defence Minister Yoav Gallant said he had ordered Gaza’s “complete siege.”

“There will be no electricity, no food, no fuel, everything is closed. We are fighting barbarians and we act accordingly. The price the Gaza Strip will pay will be a very heavy one that will change reality for generations.”

Israel’s threats triggered a response from UN Secretary General Antonio Guterres who warned that an already “extremely dire” humanitarian situation in Gaza would “deteriorate exponentially”.

“I am deeply distressed by today’s announcement that Israel will initiate a complete siege of the Gaza Strip, nothing allowed in — no electricity, food, or fuel,” the secretary-general said.

Israel’s troops were still battling to clear out Hamas gunmen who had burst across the fence from Gaza on a deadly rampage.

Fighting raged at several locations inside Israel where the fighters were still holed up after killing 700 Israelis and seizing dozens of hostages in a raid that shattered Israel’s reputation of invincibility.

Israel’s chief military spokesman said troops had re-established control of communities that had been overrun, but that isolated clashes continued as some Palestinian gunmen remained active.

Israeli air strikes have hit housing blocks, tunnels, a mosque and Hamas officials’ homes in Gaza.

“We are now carrying out searches in all of the communities and clearing the area,” chief military spokesperson Rear-Admiral Daniel Hagari said.

The shocking images of the bodies of hundreds of Israeli civilians sprawled across the streets of towns, gunned down at an outdoor disco and abducted from their homes were like nothing seen before in the decades-old Israeli-Palestinian conflict.

Israel has already responded with its heaviest ever bombardment of the Gaza Strip, killing some 500 people so far, and could be contemplating an unprecedented ground assault of the territory it abandoned nearly two decades ago.

Hagari said 300,000 reservists had already been activated in just two days.

“We have never drafted so many reservists on such a scale,” Hagari said. “We are going on the offensive.”

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Hamas says the attack is justified by the plight of Gaza under a 16-year blockade, an Israeli crackdown in the occupied West Bank that has been the deadliest in years, and a far-right Israeli government that talks of annexing Palestinian land. Israel and Western countries say nothing justifies the intentional mass killing of civilians.

The violence jeopardises US-backed moves towards normalising relations between Israel and Saudi Arabia – a security realignment that could have threatened Palestinian hopes of self determination and hemmed in Hamas’s backer Iran.

Hamas fighters were continuing to cross into Israel from Gaza, the military said, adding that between 70 and 100 gunmen have been killed in the Beeri area since Saturday.

Fighter jets, helicopters and artillery struck over 500 Hamas and Islamic Jihad targets in the Gaza Strip overnight, with targets including Hamas and Islamic Jihad command centres and the residence of a senior Hamas official, Ruhi Mashtaa.

“The price the Gaza Strip will pay will be a very heavy one that will change reality for generations,” said Defence Minister Yoav Gallant in Ofakim, one of the towns that where calm was restored after a battle with Hamas fighters who stormed through it, killing civilians and leaving with hostages.

Prime Minister Benjamin Netanyahu’s options for hitting Hamas, which controls the narrow Gaza Strip that is home to 2.3 million Palestinians, could be curtailed by concern for the many Israelis seized in the raid.

A full-scale invasion of Gaza, which Netanyahu has tried to avoid in his long years in power, could endanger the lives of the hostages.

In a statement, the Israeli Air Force said it dropped some 2000 munitions and more than 1000 tonne bombs on Gaza aimed at over 8000 targets in Gaza in the last 20 hours.

Among the targets were three rocket launchers directed at Israel, a mosque where militants were operating and 21 high-rise buildings that served militant activity.

US President Joe Biden spoke to Netanyahu for the second straight day, saying in a post on the social media platform X that he expressed “my full support for the people of Israel in the face of an unprecedented and appalling assault by Hamas terrorists.”