Israel - Israel gathered tens of thousands of troops around the Gaza Strip on Monday, as gunfights raged between Hamas and Israeli forces, two days after the Islamist militants attacked Israel.

Hundreds of people have been killed on both sides since the Iran-backed Hamas launched the massive assault at dawn on Saturday, which the Israeli army dubbed "by far the worst day" in the country's history.

This is what we know about the conflict so far.

Hundreds of Hamas militants attacked Israel from around 6:30 AM local time on Saturday, the Jewish holiday of Simchat Torah, in an assault that came 50 years after the outbreak of the 1973 Arab-Israeli war.

The Islamist group fired thousands of rockets into Israel as its militants used explosives and bulldozers to break through the fence surrounding the blockaded Palestinian enclave.

Using motorcycles, pickup trucks, motorized gliders, and speed boats, more than 1,000 militants streamed into Israel including the cities of Ashkelon, Ofakim, and Sderot.

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The gunmen carried out a bloody attack on a music festival attended by hundreds of young Israelis and foreigners near Kibbutz Reim close to Gaza.

Israel said Hamas has taken more than 100 hostages. They include an unknown number of foreigners, among them Americans.

Israel officially declared war on Hamas on Sunday.

Prime Minister Benjamin Netanyahu vowed to reduce the group's hideouts in Gaza – an impoverished enclave of 2.3 million people hemmed in by an Israeli blockade for more than 15 years – to "rubble."

He warned Gaza civilians to move away from Hamas sites – many of which are allegedly placed in civilian centers purposefully for cover – as Israel's army carried out air strikes on its positions in an operation it has dubbed "Swords of Iron."

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On Monday the army declared its forces were now "in control" of the southern communities attacked by Hamas gunmen.

The army said Monday that it struck 500 Hamas and Islamic Jihad sites overnight, after carrying out 800 previously. It has massed tens of thousands of troops and heavy armor near Gaza, with Palestinians bracing for a potential ground assault. Defence Minister Yoav Gallant on Monday ordered a "complete siege" on Gaza: "No power, no food, no gas, everything is closed. We are fighting human animals and we act accordingly."

The United Nations said more than 123,000 people have been displaced inside Gaza since the outbreak of violence.

Israel says Hamas gunmen have killed more than 700 people and wounded over 2,000 in Israeli cities, towns, and kibbutz communities.

AFP journalists have seen the bullet-riddled bodies of civilians lying on the streets in at least three locations in Israel: the city of Sderot, the nearby kibbutz of Gevim, and Zikim beach north of the Palestinian coastal enclave.

An estimated 250 people were killed at the Supernova music festival near Kibbutz Reim, according to a volunteer who helped recover the bodies.

On the Gaza side, health officials said at least 430 people had been killed and more than 2,200 wounded, taking the combined toll to more than 1,100 dead.

The United States said at least four Americans were killed, with the toll likely to rise. A British national, 12 Thais, and 10 Nepalis are among other foreigners killed in the attacks.

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

On Saturday its chief, Ismail Haniyeh, vowed 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.

Early on Sunday, Lebanon's Iran-backed Shiite militant group Hezbollah said it launched missiles and artillery shells into northern Israel "in solidarity" with the Hamas offensive.

The Israeli army retaliated to this assault with artillery fire.

Elsewhere, media outlets in Egypt said a policeman opened fire on an Israeli tour group in the northern city of Alexandria on Sunday, killing two Israelis and one Egyptian.

United Nations Secretary-General Antonio Guterres condemned "in the strongest terms" Hamas' attack on Israel and called for "diplomatic efforts to avoid a wider conflagration."

Western capitals, many of whom have designated Hamas a "terrorist" organization, also condemned the Islamist group's assault.

President Joe Biden said that the United States' support for its key ally Israel was "rock solid and unwavering." Biden said on Sunday that he had ordered "additional support" for Israel and moved an aircraft carrier group to the eastern Mediterranean.

European Union chief Ursula von der Leyen said: "I unequivocally condemn the attack carried out by Hamas terrorists against Israel."

On Monday Iran rejected allegations that it had a role in Hamas' attack after the Wall Street Journal reported that it had helped to plan it and gave the Palestinian group the "green light" for the assault.

Turkish President Recep Tayyip Erdogan, who strongly supports the Palestinian cause, urged both sides "to support peace."