President Joe Biden urged Benjamin Netanyahu on Wednesday to follow the rules of war after the Israeli prime minister vowed to destroy Hamas following the Palestinian militants' brutal attack.
The 80-year-old president, who has dispatched a US aircraft carrier to the region in a show of support for Israel, also warned Hamas-backer Iran to "be careful."
Biden told a gathering of US Jewish community leaders at the White House that the attacks on Israel in which 1,200 people were killed was the "deadliest day for Jews since the Holocaust."
But as Israel responded with air strikes on Gaza which Palestinian officials say have killed more than 1,000 people, Biden said he had spoken earlier Wednesday to Netanyahu.
The US president said he had known "Bibi" Netanyahu for 40 years and they had a "very frank relationship, I know him well."
"And the one thing that I did say is that it is really important that Israel, with all the anger and frustration... that exists, is that they operate by the rules of war," Biden said.
"And there are rules of war."
The remarks were the first time Biden had made any kind of call for restraint over Israel's response to what he called the "sheer evil" of the Hamas attacks on Saturday.
Netanyahu had earlier vowed the complete destruction of the militant group.
"Every Hamas member is a dead man," the veteran right-wing Israeli leader said, likening them to the Islamic State group.
- 'Be careful' -
But Biden also had a stern warning for Iran, which backs Hamas and Lebanon's Hezbollah, and is a long-term foe of both Israel and the United States.
The president recalled that Washington had moved the world's largest aircraft carrier, the USS Gerald R. Ford, and its fleet into the region. US officials said another carrier would soon be available if needed.
Biden said he had "made it clear to the Iranians. Be careful."
US Secretary of State Antony Blinken headed earlier Wednesday to Israel in a show of solidarity with Israel.
While fears have been growing of a regional conflagration, US officials said earlier they still had not seen any intelligence to show Iran was involved in planning or preparing the Hamas attacks.
At least 22 Americans were killed in the Hamas attacks, while an unknown number of US citizens are among an estimated 150 people -- including women, children and the elderly -- who were taken hostage by the militants.
The Hamas assault on Israel has raised fears of domestic attacks in the United States, and compounded concerns about a resurgence of anti-Semitism across the United States.
Biden said on Tuesday that US authorities were on alert.
"In cities across the United States of America, police departments have stepped up security around centers... of Jewish life," Biden said.
US National Security Advisor Jake Sullivan also said on Tuesday that the president would meet with his domestic security advisors later in the week.
In August, a US shooter was sentenced to death for the 2018 massacre of 11 Jewish worshippers at the Tree of Life synagogue in Pittsburgh -- the deadliest anti-Semitic attack on American soil.