Israel massed 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:

How it unfolded

The army said 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 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 motorbikes, pickup trucks, motorised gliders and speed boats, more than 1,000 militants streamed into Israel, including the cities of Ashkelon, Ofakim and Sderot.

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.

How Israel is responding

Israel officially declared war on Hamas on Sunday.

Prime Minister Benjamin Netanyahu vowed to reduce to "rubble" 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.

He warned Gaza civilians to move away from Hamas sites as Israel's army said it carried out air strikes on its positions in an operation it has dubbed "Swords of Iron".

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

The army said Monday it struck 500 Hamas and Islamic Jihad sites overnight, after carrying out 800 previously.

It has massed tens of thousands of troops and heavy armour near Gaza, with Palestinians bracing for a potential ground assault.

Defence Minister Yoav Gallant on Monday ordered a "complete siege" on Gaza, meaning "no electricity, no food, no water, no gas".

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

More than 1,100 killed

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, a volunteer who helped recover the bodies for the said.

"They butchered people in cold blood in an inconceivable way," said Moti Bukjin, a spokesman for the humanitarian organisation Zaka.

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 Briton, 12 Thais and 10 Nepalis are among other foreigners killed.

What Hamas says about the offensive

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

Its chief Ismail Haniyeh on Saturday 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 said it retaliated 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.

How the world has reacted

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" organisation, also condemned the Islamist group's assault.

President Joe Biden said the United States' support for its key ally Israel was "rock solid and unwavering". The White House said on Sunday 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."

Iran on Monday rejected allegations 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".