Israel hammered the Gaza Strip on Tuesday with the fiercest air strikes in its 75-year conflict with the Palestinians, razing entire districts despite a threat from Hamas militants to execute a captive for each home hit.

Across the barrier wall enclosing the coastal enclave, Israeli soldiers collected the last of the dead four days after Islamist Hamas gunmen rampaged through towns in the deadliest attack in Israel’s history.

Israel has vowed to take “mighty revenge”, calling up hundreds of thousands of reservists and placing Gaza, crowded home to 2.3 million people, under total siege.

Israel’s embassy in Washington said the death toll from the weekend Hamas attacks had surpassed 1,000, dwarfing all modern Islamist attacks on the West except for 9/11.

The victims were overwhelmingly civilians, gunned down in homes, on streets or at an outdoor dance party. Scores of Israelis and some foreigners were captured and taken to Gaza as hostages, some paraded through the streets.

Gaza’s health ministry said Israel’s retaliatory airstrikes had killed at least 830 people and wounded 4,250. The strikes intensified on Tuesday night, shaking the ground and sending more columns of smoke and flames into the sky.

The United Nations said more than 180,000 Gazans had been made homeless, many huddling on streets or in schools.

At the morgue in Gaza’s Khan Younis hospital, bodies were laid on the ground on stretchers with names written on their bellies. Medics called for relatives to pick up bodies quickly because there was no more space for the dead.

A municipal building was hit while being used as an emergency shelter. Survivors there spoke of many dead.

“No place is safe in Gaza, as you see they hit everywhere,”

said Ala Abu Tair, 35, who had sought shelter there with his family after fleeing Abassan Al-Kabira near the border.

Radwan Abu al-Kass, a boxing instructor and father of three, said he had been one of the last to evacuate his five-storey building in the Al Rimal district after the area came under attack. He finally left when a missile hit the building, which was destroyed by a bigger strike after he got out.

Two members of Hamas’ political office, Jawad Abu Shammala and Zakaria Abu Maamar, were killed in an air strike in Khan Younis in southern Gaza, a Hamas official said. The Israeli military said they had been struck overnight.

They were the first senior Hamas members killed since Israel began pounding the enclave. Israel said Abu Shammala had led a number of operations targeting Israeli civilians. He was a member of the Hamas politburo in charge of economic affairs.

Three Gaza journalists were killed while reporting outside a building, bringing the number of journalists killed to six. The Palestinian Foreign Ministry said Israeli strikes had since Saturday destroyed more than 22,600 residential units and 10 health facilities and damaged 48 schools.

UN High Commissioner for Human Rights Volker Turk, who denounced the Hamas attacks, said civilians had been harmed in Israeli strikes on tower blocks, schools and UN buildings.

“International humanitarian law is clear: the obligation to take constant care to spare the civilian population and civilian objects remains applicable throughout the attacks,” he said.

In Israel, there has still been no complete official count of the dead and missing from Saturday’s attacks. In the southern town of Be’eri, where more than 100 bodies have been retrieved, volunteers in yellow vests and face masks carried the dead out of homes on stretchers.

A long, wide trail of blood wound along the floor of a house where bodies had been dragged out by militants into the street from a blood-soaked kitchen strewn with overturned furniture.

“The thing I want the most is to wake up from this nightmare,” said Elad Hakim, a survivor from an outdoor music festival where Hamas had killed 260 partygoers at dawn.

Amid the burned-out houses of the Kfar Aza kibbutz, bodies of Israeli residents and Hamas militants lay on the ground beside scattered furniture and torched cars. Israeli soldiers went from house to house to take away the dead. The stench of corpses hung heavy in the air.

“You see the babies, the mothers, the fathers, in their bedrooms, in their protection rooms and how the terrorist kills them. It’s not a war, it’s not a battlefield. It’s a massacre,” said Israeli Major General Itai Veruv, escorting journalists at the scene.

“It’s something we used to imagine from our grandfathers, grandmothers in the pogrom in Europe and other places.”

Soldiers were still securing the paths of the kibbutz as bursts of gunfire and explosions could be heard in the distance. Jets roared above and smoke could be seen rising from Gaza. Sirens warned of incoming rockets intercepted overhead.

Israel’s next move could be a ground offensive into Gaza, territory it left in 2005, after 38 years of occupation, and has kept under blockade since Hamas seized power there in 2007. The siege it announced on Monday would keep out even food and fuel.

Israel also struck the border gate inside the sole crossing from Gaza into Egypt to the south.

Israel was caught so completely off guard by Saturday’s Hamas assault that it took more than two days to finally seal off the multi-billion-dollar, high-tech barrier wall that was meant to have been impenetrable.

Israeli leaders now must decide whether to constrain their retaliation to safeguard the hostages now hidden in Gaza. Hamas spokesperson Abu Ubaida issued a threat on Monday that one Israeli captive would be killed for every Israeli bombing of a civilian house without warning – and to broadcast such killings.

The new war tore up the plans of diplomats at a crucial juncture, when Israel was on the verge of an agreement to normalise relations with the richest Arab power, Saudi Arabia.

Western countries have backed Israel. Arab cities have seen demonstrations in support of the Palestinians. Iran, Hamas’s patron, celebrated the attacks but denied a direct role.

“We kiss the hands of those who planned the attack on the Zionist regime,” Iranian Supreme Leader Ali Khamenei said in a televised speech wearing a Palestinian scarf, though he said accusations Tehran was behind it were false.

• A Reuters report