Gaza's Health Ministry said that at least 198 people had been killed and a further 1,610 were "wounded with different injuries" following Israeli air strikes on the Palestinian enclave.
The strikes were a response by Israel to the surprise assault on Israel by Hamas militants. Prime Minister Benjamin Netanyahu said their "enemy will pay a price like they have never known before."
Observers have been wondering whether Israeli forces will launch a ground invasion against Gaza or whether the response will be limited to strikes.
US pledges 'unwavering' support for Israel
The United States underlined its full support for Israel following the rocket strikes and incursion by Hamas militants in Israeli border towns on Saturday.
"Our commitment to Israel's right to defend itself remains unwavering," Defense Secretary Lloyd Austin said in a statement.
He added that the Pentagon would work "to ensure that Israel has what it needs to defend itself and protect civilians from indiscriminate violence and terrorism."
The White House mirrored Austin's comments, with National Security Council spokesperson Adrienne Watson saying: "The United States unequivocally condemns the unprovoked attacks by Hamas terrorists against Israeli civilians. There is never any justification for terrorism."
"We stand firmly with the government and people of Israel and extend our condolences for the Israeli lives lost in these attacks. National Security Advisor Jake Sullivan has spoken to Israeli National Security Advisor Tzachi Hanegbi and we remain in close contact with our Israeli partners," Watson added.