US President Donald Trump said most NATO allies had informed Washington they did not want to participate in the military operation against Iran, and declared that the US did not need their assistance, in a post on Truth Social on March 17.

The statement came hours after his own counterterrorism chief, Joseph Kent, resigned in protest at the war, and a day after Trump had called on allied nations to send warships to help reopen the Strait of Hormuz.

"I am not surprised by their action, however, because I always considered NATO, where we spend hundreds of billions of dollars per year protecting these same countries, to be a one way street," Trump wrote.

Trump claimed the US had "decimated Iran's military," saying its navy, air force, air defences and radar were destroyed and that its leaders "at virtually every level, are gone."

He said the US no longer needed or desired NATO's help, adding: "WE NEVER DID! Likewise, Japan, Australia, or South Korea."

"WE DO NOT NEED THE HELP OF ANYONE!" Trump wrote.

The post marks a sharp reversal from March 16, when Trump said he had demanded around seven countries send warships to police the Strait of Hormuz. The pivot suggests allied refusals prompted the change in tone.

The war, which began on February 28 with US-Israeli strikes on Iranian territory, has killed more than 1,400 people in Iran and at least 13 US service members. Iran's retaliatory strikes have hit Gulf states including Saudi Arabia, the UAE, Kuwait and Bahrain.