Gaza - Palestinians and Israel are at war after a barrage of rockets was fired and fighters from the Gaza strip have broken through the walled-off enclave in a major escalation of the conflict.

"Citizens of Israel, we are at war," the Israeli leader said in a video message from military headquarters in Tel Aviv.

Israel's Defense Minister Yoav Gallant added that "Troops are fighting against the enemy at every location."

AFP journalists said Israel's military began air strikes on Gaza, following the rocket barrage from inside the territory which is sealed off from Israel by a militarized border barrier.

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Rockets had earlier streamed across the sky repeatedly after the first launches from multiple locations across the Palestinian territories from 6:30 AM Friday (11:30 PM EDT Friday), AFP journalists in Gaza City reported.

The armed wing of Hamas, which controls Gaza, said it was behind the aerial assault, saying its militants had launched thousands of rockets and its fighters seized an Israeli tank.

"We decided to put an end to all the crimes of the occupation (Israel). Their time for rampaging without being held accountable is over," the Ezzedine al-Qassam Brigades said. "We announce Operation Al-Aqsa Flood and we fired, in the first strike of 20 minutes, more than 5,000 rockets."

Hamas called on "the resistance fighters in the West Bank" as well as "our Arab and Islamic nations" to join the battle, in a statement posted on Telegram.

Several US citizens have been likely killed and others are missing following the Hamas Palestinian extremist organization's attack on Israel on Saturday, US Secretary of State Antony Blinken said on Sunday.

"We have reports that several Americans were killed. We're working overtime to verify that," Blinken told CNN on Sunday.

"At the same time, there are reports of missing Americans and there again, we’re working to verify those reports," he added. Blinken also confirmed these details to the NBC television network.

The death toll in Israel after the surprise attack by Hamas has surged to more than 600, while over 100 people were being held as "prisoners" by the militant group, the Israel government said.

The government's press office posted the latest figures on its Facebook.

This "is not a final number," an official told AFP, adding that more than 2,000 people had been wounded including 200 who were in "critical condition".

Israel said on Sunday it would launch attacks against Hamas in response to the surprise large-scale attacks by the militant group in the Gaza Strip.

Israel's security council had approved steps to bring about the "destruction of the military and governmental capabilities" of Hamas, the office of Israel's Prime Minister Benjamin Netanyahu said on Sunday.

The supply of electricity, fuel, and goods into Gaza will be halted, the office said.

Netanyahu wrote on X that Israel was "embarking on a long and difficult war."

An offensive had started "and it will continue without reservation and without respite – until the goals are achieved," he said.

Already, more than 300 people have been killed in sweeping airstrikes on Gaza City that reduced buildings – among them, the 14-story Palestine Tower – to rubble. Israel also reported at least 300 deaths in the Hamas attacks.

The US "unequivocally" condemned attacks by "Hamas terrorists" against Israel and pledged its support.

President Joe Biden was briefed on "the appalling Hamas terrorist attacks in Israel," the White House said in a statement, adding that he was in close contact with Israeli partners as the situation developed.

"We stand firmly with the Government and people of Israel and extend our condolences for the Israeli lives lost in these attacks," National Security Council spokesperson Adrienne Watson said.

Defense Secretary Lloyd Austin reaffirmed the "unwavering" commitment to Israel.

"Over the coming days the Department of Defense will work to ensure that Israel has what it needs to defend itself and protect civilians from indiscriminate violence and terrorism," Austin said.

At least 198 people were killed in Gaza, the health ministry in the Palestinian enclave said, after Israel responded to a shock military operation by militants by launching airstrikes.

The ministry also said that 1,610 people were wounded.

Meanwhile, Israeli media is reporting that at least 100 Israelis were killed in the day's attacks.

Hamas also claims to have taken "a big number of Israeli captives, among them senior officers," as fighting continues in villages and settlements where Palestinian fighters have broken through the Gaza–Israel barrier.

Hundreds of residents fled their homes in eastern Gaza to move away from the border with Israel, an AFP correspondent said.

Men, women and children carrying blankets and food left their homes, mostly in the northeastern part of the territory, the reporter said.

Israel's military said Hamas launched "massive shooting of rockets," while at the same time "terrorists infiltrated into Israeli territory in a number of different locations."

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Hamas "will face the consequences and responsibility for these events," it said in a statement.

A regional council for Israeli communities northeast of Gaza said its president was killed in an exchange of fire with attackers from Gaza.

Israel has imposed a crippling blockade on Gaza since 2007 after Hamas took power and has expanded illegal settlements in the West Bank.

Palestinian militants and Israel have fought several devastating wars since.

The latest violence follows increasingly violent crackdowns and raids on Palestinians, including repeated flashpoints at the Al-Aqsa Mosque in Jerusalem.

So far this year at least 247 Palestinians, 32 Israelis and two foreigners have been killed in the conflict. The vast majority of fatalities have occurred in the West Bank, which has been occupied by Israel since the 1967 Arab-Israeli War.

There has been a rise in army raids and Israeli settler violence against Palestinians and their property, with Palestinian targeting Israelis in response.