"The United States unequivocally condemns the unprovoked attacks by Hamas terrorists against Israeli civilians," National Security Council spokeswoman Adrienne Watson said in a statement, adding that "there is never any justification for terrorism."

National Security Advisor Jake Sullivan has spoken to Israeli National Security Advisor Tzachi Hanegbi and will remain in close contact with Israeli partners over the operation, according to the statement.

"We stand firmly with the Government and people of Israel and extend our condolences for the Israeli lives lost in these attacks," Watson added.

In a separate statement, Defense Secretary Lloyd Austin reaffirmed the United States' "unwavering" commitment to Israel's right to defend itself.

"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.

The Palestinian militant group Hamas launched an air, sea and land attack on Israel Saturday.

Israel said it was at war with Hamas following the surprise dawn attack from the blockaded Gaza Strip that, according to medics, had so far killed at least 40 people on its territory.