JERUSALEM — Thousands have died and the toll was continuing to climb dramatically five days after Palestinian militants launched a surprise attack on Israel, which has responded with massive shelling of Gaza.

Israel said it had recaptured Gaza border areas that were invaded by Hamas militants in Saturday's assault and was continuing heavy bombardments of the Palestinian enclave from the air, land, and sea.

In the north, Israel's military also said it had responded to artillery fire and shelling from Lebanon and Syria.

This is what we know about the conflict so far.

How the assault unfolded

Hundreds of Hamas militants attacked Israel from around 6:30 am (0330 GMT) on Saturday, the Jewish holiday of Simchat Torah, in an assault that came 50 years and a day after the outbreak of the 1973 Arab-Israeli war.

The Iran-backed Islamists fired thousands of rockets from Gaza into Israel and used explosives and bulldozers to break through the security fence around the blockaded Palestinian enclave.

Using motorbikes, pickup trucks, motorized gliders and speed boats, the militants streamed into Israel, targeting cities including Ashkelon, Ofakim and Sderot.

The gunmen unleashed a massacre at a music festival attended by hundreds of young Israelis and foreigners near Kibbutz Reim, close to Gaza.

Israel said Hamas took around 150 hostages. They include at least 11 Thais, two Mexicans, and an unknown number of Americans and Germans.

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How Israel is responding

Israel declared war on Hamas on Sunday and has since unleashed thousands of air strikes on the militia's positions inside Gaza.

Defence Minister Yoav Gallant on Monday ordered a "complete siege" of Gaza, meaning "no electricity, no food, no water, no gas", in a move decried by the United Nations as a possible violation of international law.

The military has massed tens of thousands of troops and heavy armor near Gaza, and called up 300,000 reservists for a campaign it has dubbed "Swords of Iron."

Prime Minister Benjamin Netanyahu has warned that his country's military campaign was only the start of a sustained war, and has vowed to reduce to "rubble" Hamas's hideouts in Gaza.

By Tuesday evening, the United Nations said that the air strikes had forced more than 260,000 people to flee their homes in the impoverished territory of 2.3 million people, which has been hemmed in by an Israeli-Egyptian blockade for more than 15 years.

The Israeli military meanwhile said Tuesday that it had largely reclaimed the embattled south and the border around Gaza, and dislodged holdout Hamas fighters from more than a dozen towns and kibbutzim.

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But late Tuesday in the southern Israeli city of Ashkelon, troops backed by helicopters and drones exchanged fire with several militants, leaving three fighters dead, the army said.

Israel also faced the threat of a multi-front war after days of clashes with the Iran-backed Shiite militant group Hezbollah on the northern border with Lebanon.

There was an exchange of fire between Israel and forces in Syria on Tuesday, after Israel's military said munitions were fired toward the Golan Heights it has occupied since 1967.

Three Israeli strikes have also hit the Rafah crossing with Egypt, the only entry point into Gaza not controlled by Israel.

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Death toll mounts

In total, the war has left more than 3,600 dead among Israeli and Palestinian civilians, soldiers, and combatants.

Israel says Hamas gunmen have killed more than 1,200 people and wounded over 2,700 in Israeli cities, towns, and kibbutz communities.

In the kibbutz of Beeri alone, Hamas gunmen killed more than 100 people, a volunteer who helped recover the bodies told AFP.

An estimated 270 people were killed at the Supernova music festival near Kibbutz Reim.

On Tuesday, the Israeli military said it had found the bodies of 1,500 Islamist militants in the southern areas that it recaptured.

Eighteen Thais, 14 US citizens, and 10 Nepalis are among foreigners from more than a dozen countries killed.

On the Gaza side, health officials said 900 people had been killed and more than 4,000 wounded.

Hamas's armed wing, the Ezzedine al-Qassam Brigades, on Monday said Israeli air strikes on Gaza had killed "four of the enemy's prisoners", in a claim that could not be independently verified.

Eight Palestinian journalists have been killed in Israeli air strikes, according to media unions and officials.

Unrest has also broken out in the occupied West Bank, where 15 Palestinians have died since Saturday.

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What Hamas says about the attack

Hamas said it fired 5,000 rockets in an offensive it branded "Operation Al-Aqsa Flood."

On Monday it threatened to start killing hostages every time Israel launches a strike on a civilian target without warning.

It has ruled out negotiating a prisoner swap with Israel as "the military operation is still ongoing."

Its chief Ismail Haniyeh vowed Saturday to press ahead with "the battle to liberate our land and our prisoners languishing in occupation prisons".

Hamas has called on "resistance fighters in the West Bank" as well as in "Arab and Islamic nations" to join the battle.

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How the world has reacted

The United States has vowed "rock solid and unwavering" support for Israel, with President Joe Biden condemning the attacks by Hamas as "sheer evil" and so far showing no sign of calling for restraint by its key ally.

Biden said Washington was "surging assistance" for Israel, including missiles for its Iron Dome interceptor system, while the largest US aircraft carrier has been ordered to move closer to Israel.

Biden said he was ready to move "additional assets" if needed to deter Israel's adversaries.

Washington is also deploying hostage experts to Israel to help but says it is so far not sending troops.

A joint statement by the United States, Britain, France, Germany, and Italy condemned the attack and stressed their full support for Israel.

They said they "recognize the legitimate aspirations of the Palestinian people" but said Hamas "offers nothing for the Palestinian people other than more terror and bloodshed".

Iran has meanwhile celebrated the Hamas assault but has repeatedly denied allegations it had a role in it.

Lebanon's Hezbollah, a Hamas ally, said it launched projectiles into northern Israel and Israel hit back with shelling on Tuesday, in the third successive day of cross-border fire.

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© Agence France-Presse