The US says it is moving an aircraft carrier, ships and jets to the eastern Mediterranean and will also give Israel additional equipment and ammunition.
It follows the Hamas attack on southern Israel, which President Biden called an “unprecedented and appalling assault”.
A US National Security spokesperson said several US citizens were among the dead.
Israel says more than 700 people have been killed and 100 kidnapped.
In Gaza, more than 400 people have been killed following retaliatory Israeli air strikes, according to Palestinian officials.
US Defence Secretary Lloyd Austin said the USS Gerald R Ford aircraft carrier, a missile cruiser and four missile destroyers were heading to the region. He said fighter jets would also be sent.
Further military aid to Israel would be sent in the coming days, the White House said, adding that the US was working to ensure Israel’s enemies did not try to seek advantage from the situation.
The large deployment reflects American concerns that the conflict between Israel and Hamas could draw in other parts of the region.
In particular, the US is eager to prevent Lebanon’s powerful Hezbollah movement from joining the conflict. It is backed by Iran, which also funds and arms Hamas.
Iran’s President Ebrahim Raisi has expressed support for the Hamas attack, saying Israel needed to be held to account for endangering the region.
Hamas has said assistance from Iran helped it carry out its attack, which involved rockets, drones and militants on paragliders and saw hundreds of fighters break through Israeli border fortifications around the Gaza Strip.
US Secretary of State Antony Blinken said the US had not seen evidence of direct Iranian involvement, but that Iran had been helping the Gaza-based group for years.
“Hamas wouldn’t be Hamas without the support that it’s gotten over many years from Iran. We haven’t yet seen direct evidence that Iran was behind this particular attack or involved. But the support over many years is clear,” he told US TV.
Hundreds of people are still reported missing. Among them is 23-year-old American-Israeli Hersh Golberg-Polin, who was at a music festival in the desert which was stormed by militants.
His parents told the Jerusalem Post they received two messages from him reading: “I love you” and “I’m sorry.”
Reports have started to emerge of other missing North Americans too, including peace activist Vivian Silver.
A friend of the 75-year-old Canadian told The Canadian Jewish News that Ms Silver had called her sister on Saturday morning to say Palestinian militants were at her door.
“There was screaming and yelling and Vivian was fighting and then the phone went dead,” he said.
Israel’s ambassador to the US, Michael Herzog, told CBS News that he understood Americans were among the soldiers and civilians abducted in southern Israel but did not have details.
The US sends billions of dollars of military aid to Israel, a close ally, each year. Since World War Two, Israel has been the largest overall recipient of US foreign aid.
The UN Security Council is due to meet in New York shortly to discuss the violence in Israel and Gaza.
BBC News
