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”.
The US was also working to verify reports its citizens were among those killed and taken captive, Secretary of State Antony Blinken said.
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. US 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 that Israel’s enemies would not try to seek advantage from the situation.
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.
At a UN Security Council meeting, Iran denied involvement in the attack on Israel, Reuters reported.