The Islamist militant Hamas movement threatened to execute an Israeli captive every time Israel bombs a Palestinian home without warning, as Israel called up an unprecedented 300,000 reservists and imposed a blockade on the Gaza Strip, raising fears it planned a ground assault.

The violence, which has claimed more than 1,500 lives, prompted international declarations of support for Israel after a devastating weekend attack by Hamas, and appeals for an end to the fighting and protection of civilians.

Israeli TV channels said the death toll from the Hamas attack had climbed to 900 Israelis, with at least 2,600 injured, and dozens taken captive. Among the Israeli dead were 260 mostly young people gunned down at a desert dance party. A day later dozens of survivors were still emerging from hiding. The site was littered with wrecked and abandoned cars.

Gaza's Health Ministry said at least 687 Palestinians had been killed and 3,726 wounded in Israeli air strikes on the blockaded enclave since Saturday. Apartment blocks and hospitals were among the sites attacked, according to media reports.

Fighting continued into the night on Monday. The Israeli military said it struck targets in the Gaza Strip from the sea and air, including a weapons depot it said belonged to Islamic Jihad and Hamas targets along Gaza's coast line.

Witnesses said several Hamas security headquarters and ministries were hit, and the strikes destroyed some roads and houses. Israel also bombed the headquarters of the private Palestinian Telecommunication Co., which could affect landline telephone, internet and mobile phone services.

Hamas spokesman Abu Ubaida issued the threat on Monday to kill Israelis among the dozens held captive after the surprise attack on Saturday morning. He said Hamas would execute an Israeli captive for every Israeli bombing of a civilian house without warning.

There was no immediate response from the Israeli military to that threat. Israeli Foreign Minister Eli Cohen said more than 100 people had been taken captive by Hamas during the deadly cross-border incursion over the weekend.

Palestinians reported receiving calls and mobile phone audio messages from Israeli security officers telling them to leave areas mainly in the northern and eastern territories of Gaza, and warning that the army would operate there.

In Israel's south, scene of the Hamas attack, Israel's chief military spokesman said troops had re-established control of communities inside Israel that had been overrun, but isolated clashes continued as some gunmen remained active.

The announcement that 300,000 reservists had been activated in just two days added to speculation that Israel could be contemplating a ground assault of Gaza, a territory it abandoned nearly two decades ago.

"We have never drafted so many reservists on such a scale," chief military spokesperson Rear-Admiral Daniel Hagari said. "We are going on the offensive."

INTERNATIONAL RESPONSE

The impact was felt internationally, with governments from Italy, Thailand and Ukraine also reporting that their citizens had perished. More than 10 Britons are feared dead or missing, the BBC reported on Monday, citing an official source, although foreign minister James Cleverly said he would not speculate on the number of British or dual nationals in the area.

In Washington, President Joe Biden announced that at least 11 Americans had been killed and it was likely U.S. citizens were among those held hostage.

"I have directed my team to work with their Israeli counterparts on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts," Biden said in a statement.

As Israel conducted intense retaliatory strikes on Gaza, Defence Minister Yoav Gallant drew international condemnation by announcing a tightened blockade to prevent food and fuel from reaching the strip, home to 2.3 million people.

"Depriving the population in an occupied territory of food and electricity is collective punishment, which is a war crime," Omar Shakir, Israel and Palestine Director at Human Rights Watch, said in a statement.

At least 14 Palestinians were killed late on Monday in an Israeli strike on a house in Beit Lahia, in the northern Gaza Strip, Hamas-affiliated media reported. Six others were killed and others were injured following an Israeli strike on a house in the southern Gaza city of Khan Younis, the reports said.

Reuters was not immediately able to confirm the reports. The Israeli military had no immediate comment.

As it rained, explosions and lightning lit the skies, and the sound of bombings mixed with thunder.

U.N. Secretary-General Antonio Guterres said some 137,000 people were taking shelter with UNRWA, the U.N. agency that provides essential services to Palestinians.

Washington - which provides Israel with $3.8 billion in military assistance each year - said it was sending in fresh supplies of air defenses, munitions and other security assistance to Israel, a senior U.S. defense official said on Monday.

The British, French, German, Italian and U.S. governments issued a joint statement on Monday recognizing the "legitimate aspirations" of the Palestinian people, and supporting equal measures of justice and freedom for Israelis and Palestinians alike.

They also said that would remain "united and coordinated" to ensure Israel can defend itself.

Egyptian President Abdel Fattah al-Sisi and his Turkish counterpart Tayyip Erdogan called on Hamas and Israel to immediately end violence and protect civilians, the Egyptian presidency said.

Qatari mediators held urgent calls to try to negotiate freedom for Israeli women and children seized by Hamas in exchange for the release of 36 Palestinian women and children from Israeli prisons.

FEARS OF WIDER CONFLICT

The prospect that fighting could spread to other areas alarmed the region and world. Israeli troops "killed a number of armed suspects that infiltrated into Israeli territory from Lebanese territory," the military said, adding helicopters "are currently striking in the area."

Lebanese armed group Hezbollah fired a salvo of rockets onto northern Israel on Monday in response to at least three of its members being killed in Israeli shelling on Lebanon. Israel said one of its deputy commanders was killed in an earlier cross-border raid from Lebanon.

Fears of a widening conflict meant more volatility for investors. Oil prices jumped more than 4%, gold gained and the U.S. dollar edged up against the euro. Major international air carriers suspended or reined in flight services to or from Tel Aviv.

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