US Secretary of State Antony Blinken has arrived in Israel in a show of support after the weekend's deadly attacks by Hamas.
Mr Blinken is expected to seek the release of hostages kidnapped by Hamas, some of whom are American.
He will also urge restraint and seek safe passage for Gaza's civilians ahead of a possible Israeli ground offensive.
At least 1,200 Israelis died in the attack, and over 1,100 Palestinians have died in air strikes on Gaza.
Some 25 US citizens are also known to have died in Israel.
Hamas, which is designated a terrorist organisation by the US, took up to 150 people hostage when its fighters breached the border fence in several places in the Gaza Strip on Saturday and launched the most serious cross-border attack Israel has faced in a generation.
Mr Blinken met Prime Minister Benjamin Netanyahu on Thursday, where he was thanked for the US support.
During a joint press conference between the two leaders, Mr Blinken told the nation that "you may be strong enough on your own to defend yourself, but as long as America exists you will never, ever have to".
He also stressed the importance of taking "every possible precaution" to avoid harming more civilians as fighting continues.
He condemned the deaths caused by the "heinous attacks" by Hamas, adding that at least 25 American citizens were killed.
Mr Netanyahu said that Mr Blinken's visit to Israel was a "tangible example of America's unequivocal support of Israel".
"President Biden was absolutely correct in calling this sheer evil," he added, referring to the unprecedented attack on Saturday.
Mr Blinken is also expected to meet Israeli President Isaac Herzog on Thursday.
On Friday he will meet Palestinian President Mahmoud Abbas and Jordan's King Abdullah in the Jordanian capital Amman.
US President Joe Biden earlier said he had made it clear to Mr Netanyahu that Israel "must operate by the rules of war" in its response to Hamas' attacks.
But he said Israel had a right and a duty to respond, and called the attacks an "act of sheer evil".
Israel has said that a total blockade - including on food, fuel and other essentials - imposed on Gaza since the attacks would not be lifted until hostages are freed.
Israel has vowed to annihilate Hamas and has launched a powerful bombing campaign on the tiny enclave of 2.3 million people.
Reuters quoted an unnamed US state department official as saying there were up to 600 Americans in Gaza, some of whom wanted to leave.
Aid agencies, meanwhile, have been calling for humanitarian corridors to ease the suffering of civilians in Gaza.
