“That’s why we will continue to work towards its full implementation,” he said. “If an important country like the United States comes to a different conclusion, as appears to be the case, we will work even harder with other partners to maintain this cohesion.”
British officials have been pressing the Trump administration to respect the deal. Mrs. May called Mr. Trump to emphasize that it was “vitally important for regional security.”
Boris Johnson, the British foreign secretary, has also tried to make that point to his American counterpart, Secretary of State Rex W. Tillerson, saying the deal was “making the world a safer place,” The Daily Telegraph reported.
Under the terms of the deal, Iran agreed to accept severe limits on its ability to enrich uranium fuel for 15 years, in exchange for the revocation or suspension of economic sanctions, including a European oil embargo.
The deal also imposed for the first time a verification mechanism that allowed the international community to make sure that Iran was abiding by its promises not to pursue nuclear weapons.
It was a major diplomatic triumph for the Obama administration, and has been praised by arms control groups who say it helped avert a potential conflict or nuclear arms race in the Middle East.
There are fears that the basic framework of the accord could collapse if the United States walks away. Mr. Peskov, the Kremlin spokesman, told reporters on Friday that Russia believed Iran would abandon the deal if the United States did.
And as tensions between the United States and a nuclear-armed North Korea have escalated in recent months, European leaders have seen the Iran deal as an important example of a possible diplomatic way forward.
Germany’s foreign minister, Sigmar Gabriel, highlighted the implications of the deal for North Korea in an interview on Thursday with the German newspaper group RND. He warned that an American withdrawal could set off a scramble for nuclear weapons, threatening both international security and the European-American relationship.
“Some states could understand the breakdown of the Iran deal as a signal to provide themselves with nuclear weapons as quickly as possible,” he said. “Then we would not only have North Korea as an acute problem.”