US President Joe Biden has praised American forces who he said "helped Israel take down nearly all" the drones and missiles launched by Iran on Sunday.
In a statement, he said the US had moved aircraft and warships to the region before the unprecedented attack.
"I condemn these attacks in the strongest possible terms," he added.
Israel said Iran launched hundreds of drones and missiles in its direction, the first time it has attacked Israel directly from its own territory.
It said the "vast majority" were intercepted, but there were a small number of hits including at an IDF base in southern Israel. At least one person, reported to be a young girl, was injured.
Iran earlier warned that Israel would be "punished" for a strike on its consulate in Syria on 1 April, which killed seven Iranian officers including a top commander. Israel has not confirmed or denied whether it was responsible.
"I've just spoken with Prime Minister [Benjamin] Netanyahu to reaffirm America's ironclad commitment to the security of Israel," Mr Biden said shortly after the pair held a call.
"I told him that Israel demonstrated a remarkable capacity to defend against and defeat even unprecedented attacks," he added.
Mr Biden also said he plans to convene G7 leaders on Sunday "to co-ordinate a united diplomatic response to Iran's brazen attack".
He warned Iran against attacking any US assets, adding while Iran has not done so, America "remains vigilant to all threats".
President Biden cut short a planned visit to his home state of Delaware on Saturday, travelling back to the White House to be briefed by national security officials hours before the attack.
White House National Security Council spokeswoman Adrienne Watson said Mr Biden was "in constant communication with Israeli officials, as well as other partners and allies".
Republicans in the House of Representatives, meanwhile, said they were drafting legislation to provide more aid to Israel and sanction Iran.
Two US officials told the OceanNewsUK's partner CBS that American warships shot down missiles headed towards Israel.
The UK's Royal Air Force (RAF) is also assisting in the region. A statement by the Ministry of Defence said British jets were ready to intercept attacks within range of the UK's existing missions in the Middle East.
Additional help has been deployed, the UK ministry added, including more jets and air refuelling tankers.
Iran's delegation at the UN said Tehran earlier said "the matter can be deemed concluded" but warned it would strike again if there were reprisals by Israel or the US.
The Iranian Revolutionary Guard also said in a statement that "support or participation in attacking Iran's interests will have a fierce response".
Other nations, including the UK, France and Canada, have also condemned Iran and expressed support for Israel.
