Boris Johnson is en route to the US for a meeting with officials as he attempts to persuade the administration not to scrap the Iran deal.
The foreign secretary will meet Vice President Mike Pence as he urges the US not to pull out of the agreement, which is designed to prevent Iran acquiring nuclear weapons.
His efforts are part of a joint attempt by the UK, France and Germany to keep President Donald Trump on side in the time leading up to 12 May, the deadline to persuade him to stick to the deal.
Prime Minister Theresa May also discussed the agreement with Trump in her latest call with the President.
The White House said that he "underscored his commitment to ensure that Iran never acquires a nuclear weapon".
The moves by May and Johnson follow the intervention of French President Emmanuel Macron, who urged Trump to remain in the deal last month during a visit to Washington.
Trump is expected to announce his position on the deal next week.
This comes ahead of his visit to the UK, which is currently expected to take place in July.
But the President caused offence in Britain yesterday when he claimed that an unnamed London hospital is a 'war zone' of knife crime.