President Barack Obama says the US "will not hesitate" to use force to stop Iran obtaining nuclear weapons, but says diplomacy could still succeed.
Addressing an influential pro-Israel lobby group, Obama also warned against "loose talk" of war in the dispute with Tehran.
Earlier, Israeli President Shimon Peres said Iran was "a danger to the world".
The US and its allies suspect Iran of trying to develop nuclear weapons, a claim Tehran denies.
As tension in the region rises, speculation has been mounting that Israel might launch a pre-emptive strike against Iranian nuclear sites.
Obama is also due to hold talks with Israeli Prime Minister Benjamin Netanyahu at the White House today.