Israel, reeling from the deadliest attack on its territory, declared war on Hamas Sunday as the conflict's death toll surged above 1,100 after the Palestinian militant group launched a surprise assault from Gaza.

Prime Minister Benjamin Netanyahu steeled the nation for a "long and difficult" war a day after Hamas fired a barrage of thousands of rockets at Israel and sent a wave of fighters who gunned down civilians and took at least 100 hostages.

More than 700 Israelis have been killed since Hamas launched its large-scale attack, according to the latest toll from the Israel Defense Forces (IDF) on Monday — the country's worst losses since the 1973 Arab-Israeli war.

"Israel was caught flat-footed by the unprecedented attack," said Jonathan Panikoff, director of the Scowcroft Middle East Security Initiative. "I've heard multiple comparisons to 9/11, and many Israelis are struggling to understand how this could have happened."

Gaza officials reported at least 413 deaths in the impoverished and blockaded enclave of 2.3 million people, which was hammered by Israeli air strikes on 800 targets ahead of what many feared may be a looming ground invasion.

US President Joe Biden ordered "additional support for Israel in the face of this unprecedented terrorist assault by Hamas".

A US National Security Council spokesperson confirmed that "several" Americans had been killed in the surprise attack, but did not provide further details.

US Defense Secretary Lloyd Austin said Washington "will be rapidly providing the Israel Defense Forces with additional equipment and resources, including munitions".

Austin directed the USS Gerald R. Ford aircraft carrier and group of warships to the eastern Mediterranean and said that Washington was augmenting fighter aircraft squadrons in the region.

Hamas has said the US aid amounted to "aggression" against Palestinians.

Several other countries have reported nationals killed, abducted or missing in the fighting, among them Brazil, Britain, France, Germany, Ireland, Mexico, Nepal, Thailand, and Ukraine.

Tens of thousands of Israeli forces were deployed to battle holdout Hamas fighters in the south, where the bodies of civilians had been found strewn on roads and in town centers.

"The enemy is still on the ground," said military spokesman Daniel Hagari as a second night fell after the massive opening attack.

Gun battles raged as the Israeli army sought to secure desert regions near the coastal enclave, rescue hostages and evacuate all areas near Gaza, after the cabinet formally declared war by activating Article 40 of the Basic Law.

"We'll reach each and every community until we kill every terrorist in Israel," vowed Hagari, a day after Hamas fighters surged into Israel using vehicles, boats, and motorized paragliders.

Abducted to Gaza

Shock and dismay gripped Israel after at least 100 citizens were captured by Hamas and abducted into Gaza, with images circulating on social media of bloodied hostages.

Yifat Zailer, 37, said she was horrified to see online video footage from Gaza that showed her cousin and the woman's children, aged nine months and three years.

"That's the only confirmation we have," she told AFP, her voice breaking with emotion, and adding there was no information on her cousin's husband or her elderly parents.

"After the army took control of the kibbutz, they weren't at home," she said. "We assume they were kidnapped … We want to know what their condition is, we want them to return safe. They're innocent civilians."

Israel also came under attack from the north when Lebanon's Hezbollah launched guided missiles and artillery shells Sunday "in solidarity" with the unprecedented Hamas offensive, without causing any casualties.

Israel responded with artillery strikes across the UN-patrolled border.

"We recommend Hezbollah not to come into this," said army spokesman Richard Hecht. "If they come, we are ready."

Israel was stunned when Hamas launched their multi-pronged offensive on Saturday, the Jewish Sabbath, with at least 3,000 rockets raining down as fighters infiltrated towns and kibbutz communities and stormed an outdoor rave where many revelers were shot dead.

Panicked Israelis hiding in their homes told reporters that militants were going door to door and shooting civilians or dragging them away.

'No respite'

Western capitals have condemned the attack by Hamas, which Washington and Brussels consider a terrorist group.

Israel's foes have praised the assault, including Iran whose President Ebrahim Raisi voiced support when he spoke with the leaders of Hamas and the Islamic Jihad group.

Anti-Israel protests have flared in Iraq, Pakistan, and some other majority Muslim countries, while Germany and France were among nations stepping up security around Jewish temples and schools.

In the Egyptian city of Alexandria, a police officer opened fire "at random" on Israeli tourists Sunday, killing two of them and their Egyptian guide before he was arrested.

Netanyahu — who leads a hard-right coalition government but has received pledges of support from political opponents — has vowed to turn Hamas hideouts "to rubble" and urged Palestinians there to flee.

"We are embarking on a long and difficult war that was forced on us by a murderous Hamas attack," Netanyahu wrote on X, formerly Twitter.

Israeli attacks have reduced several Gaza residential towers to rubble and destroyed a mosque in Gaza's Khan Yunis as well as the central bank.

Schools were closed and many flights were cancelled as the Tel Aviv Stock Exchange's main TA-35 index fell 6.47 percent and UEFA postponed an Israel-Switzerland Euro 2024 qualifier match.

'We will not give up'

Hamas has labeled its attack "Operation Al-Aqsa Flood" and called on "resistance fighters in the West Bank" and "Arab and Islamic nations" to join the battle.

Its attack came half a century after the outbreak of the 1973 conflict called the Yom Kippur War in Israel, sparking bitter recriminations for what was widely seen as an enormous intelligence failure.

"There was a very bad failure here," said Sderot resident Yaakov Shoshani, 70. "The Yom Kippur War was small compared to it, and I was a soldier in the Yom Kippur War."

He recalled the terror of the attack on their town near Gaza.

"I held a kitchen knife and a large screwdriver, and I told my wife that, if something happens, to make sure to read the Kaddish (prayer) over me, if you stay alive," he said. "And so we stayed close to each other at home, shut everything and turned off the lights."

Hamas chief Ismail Haniyeh has predicted "victory" and vowed to press ahead with "the battle to liberate our land and our prisoners languishing in occupation prisons".

An Israeli survivor of the attack on Sderot, Yitzhak, 67, said he now expected the army to "conquer Gaza house by house, clean the area there properly, and not leave Gaza until they get the very last rocket out of the ground."

Many Gaza residents voiced defiance.

"We will not give up, and we are here to stay," said Mohammed Saq Allah, 23. "This is our land, and we will not abandon our land."