US President Donald Trump has all but decided to withdraw from the 2015 Iran nuclear accord by May 12 but exactly how he will do so remains unclear, two White House officials and a source familiar with the administration's internal debate said.

"We'll see what happens. I'm not telling you what I'm doing but a lot of people think they know. And, on or before the 12th we'll make a decision," the president said. 

The 2015 agreement between Iran, the U.S., and five other major world powers lifted sanctions in exchange for limits on Tehran's nuclear program.

The deal marked the signature foreign policy achievement of Trump's predecessor, President Barack Obama. 

Trump has repeatedly attacked agreement. 

"It was a terrible deal. It should have never, ever been made," he said.

Under the agreement, Trump must decide by May 12 whether to renew waivers suspending US sanctions on Iran.

Re-imposing sanctions would all but end the agreement and could trigger a response from Tehran, which has suggested it could renew its nuclear enrichment programme.

US allies who signed the agreement, including France, Germany and Britain lobbied Trump to stick with the deal.  

A source told Reuters there is a chance that Trump might keep the the US in the pact to better maintain relations with its European allies.

International monitors have said Iran is in compliance with the terms of the deal.

Iran denies ever seeking nuclear weapons.