Israeli Prime Minister Benjamin Netanyahu on Sunday warned of a "long and difficult" war, as fighting with Hamas left hundreds dead on both sides after a surprise attack on Israel by the Palestinian militant group.

The conflict's bloodiest escalation in decades saw Hamas carry out a massive rocket barrage and ground, air and sea offensive Saturday that Israel's army said had killed more than 200 Israelis and wounded 1,000, while soldiers and civilians were taken hostage.

Gaza officials said intense Israeli air strikes on the coastal enclave had brought the Palestinian death toll to at least 256, with nearly 1,788 wounded.

As fighting raged Sunday, Lebanon's powerful Iran-backed Hezbollah movement said it had fired "large numbers of artillery shells and guided missiles" at Israeli positions in a contested border areas "in solidarity" with Hamas.

Israel's army had earlier said it fired artillery on southern Lebanon in response to a shot from the area without identifying the attackers.

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

"The first stage is ending at this time by the destruction of the vast majority of the enemy forces that infiltrated our territory," he added, pledging no "respite" until victory.

Overnight Israel battered the Gaza Strip with air strikes as rockets from the blockaded Palestinians territory rained on Israel.

Sunday morning gun still battles raged between Israeli forces and hundreds of Hamas fighters in multiple locations, including at the Sderot police station across the border from Gaza.

Police and Israeli army special forces "neutralized 10 armed terrorists" who were holed up inside the station, a police statement said.

The bloody air, sea and land attack launched Saturday by Hamas came half a century after the outbreak of the 1973 Arab-Israeli war, taking Israel and the world by surprise.

As the UN Security Council called an emergency meeting for Sunday, President Joe Biden voiced "rock solid and unwavering" support for the US ally and warned "against any other party hostile to Israel seeking advantage in this situation".

– Hostages and 'so many bodies' –

The Israeli army said overnight its forces were still engaged in gun battles in a string of Israel locations, in an operation labelled "Swords of Iron", as reservists were being called up.

Hamas earlier released images of several Israelis taken captive, and another army spokesman, Daniel Hagari, confirmed that soldiers and civilians had been kidnapped.

"I can't give figures about them at the moment," he said late Saturday, adding there was also a "severe hostage situation" in the Negev desert communities of Beeri and Ofakim east of Gaza.

According to Ynet Israeli news website "dozens of Israeli captives, including numerous women, children and elders, are believed to have been taken into the Gaza Strip".

The fighting prompted Israel to cut off Gaza's electricity, fuel and goods supplies, Netanyahu said.

The Islamist group started the multi-pronged attack around 6:30 am (0330 GMT) on Saturday with thousands of rockets aimed as far as Tel Aviv and Jerusalem, some bypassing the Iron Dome defense system and hitting buildings.

Hamas fighters — traveling in ground vehicles, motorized paragliders and boats — breached Gaza's security barrier and attacked nearby Israeli towns and military posts, opening fire on residents and passersby.

"Send help, please!" one Israeli woman sheltering with her two-year-old child pleaded as militants outside opened fire and tried to break into their safe room, Israeli media reported.

Bodies were strewn on the streets of the Israeli town of Sderot near Gaza and inside cars, the windscreens shattered by a hail of bullets.

"I saw many bodies, of terrorists and civilians," one man told AFP, standing beside covered corpses on a road near Gevim Kibbutz in southern Israel.

"So many bodies, so many bodies."

AFP journalists witnessed Palestinian armed men gather around a burning Israeli tank, and others driving a seized Israeli military Humvee vehicle back into Gaza, where they were met by cheering crowds.

– 'Gates of hell' –
Israeli army Major General Ghasan Alyan warned Hamas had "opened the gates of hell".

An AFP journalist in Gaza saw clouds of dust from the remains of bombed residential towers which Gaza's interior ministry said contained 100 apartments.

Israel's military said it had warned residents to evacuate before targeting the multi-story buildings used by Hamas.

The escalation follows months of rising violence, mostly in the occupied West Bank, and tensions around Gaza's border and at contested holy sites in Jerusalem.

Before Saturday, at least 247 Palestinians, 32 Israelis and two foreigners had been killed this year, including combatants and civilians, according to Israeli and Palestinian officials.

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

Its armed wing, the Ezzedine al-Qassam Brigades, claimed to have fired more than 5,000 rockets, while Hecht said Israel had counted more than 3,000 incoming rockets.

Hamas chief Ismail Haniyeh said the group was on the "verge of a great victory", vowing to press ahead with "the battle to liberate our land and our prisoners languishing in occupation prisons must be completed".

– 'Dangerous precipice' –

Air raid sirens wailed across southern and central Israel, as well as in Jerusalem on Saturday, and there were major disruptions at Tel Aviv airport where many carriers canceled flights.

Israel said schools would remain closed on Sunday which marks the start of the week.

Hamas took control of Gaza in 2007, leading to Israel's crippling blockade of the impoverished enclave of 2.3 million people.

Israel and Hamas have since fought several wars. The last major military exchange, in May, killed 34 Palestinians and one Israeli.

Violence also erupted across the West Bank, including annexed east Jerusalem, with five Palestinians killed and 120 wounded in clashes with Israeli forces and settlers, Palestinian medical services said.

Countries around the world condemned the wave of attacks by Hamas, which Israel, the United States and European Union consider a terrorist group.

European Commission President Ursula von der Leyen called the attack "terrorism in its most despicable form".

But Hamas drew support from other foes of Israel, with Iran's supreme leader declaring he was "proud".

UN Middle East peace envoy Tor Wennesland warned of "a dangerous precipice" and called on all sides to "pull back from the brink".

(Rosie Scammell with Adel Zaanoun in Gaza)

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© Agence France-Presse