Speaking to US network Fox News ahead of meetings meeting in Washington with US administration officials, the British Foreign Secretary said Trump was "right to see flaws" in the deal but "Plan B does not seem to be to me particularly well-developed at this stage."

Trump has threatened to withdraw from the agreement when it comes up for renewal on May 12, demanding US allies "fix the terrible flaws" in it or he will re-impose sanctions on Iran that were eased under the historic accord.

"The president has a legitimate point," Johnson told Fox, a favorite among conservatives. "He set a challenge for the world."

"We think you can be tougher on Iran, address the concerns of the president and not throw the baby out with the bath water, not junk a deal."

The nuclear deal was struck in 2015 among Iran and Britain, China, France, Germany, Russia and the United States, then led by Barack Obama.

Under the pact, international sanctions were eased in return for verifiable limits on Iran's nuclear program, but Iran says it is not reaping the rewards despite complying with the deal.

Johnson's comments echoed his call to preserve the deal in an op-ed piece in The New York Times.

"At this delicate juncture, it would be a mistake to walk away from the nuclear agreement and remove the restraints that it places on Iran," Johnson wrote in the piece.

He argued that inspectors from the International Atomic Energy Agency have been granted extra powers to monitor Iran's nuclear facilities, "increasing the likelihood that they would spot any attempt to build a weapon."

"Now that these handcuffs are in place, I see no possible advantage in casting them aside. Only Iran would gain from abandoning the restrictions on its nuclear program," Johnson wrote.

He added: "I believe that keeping the deal's constraints on Iran's nuclear program will also help counter Tehran's aggressive regional behavior. I am sure of one thing: every available alternative is worse. The wisest course would be to improve the handcuffs rather than break them."