Mr. Trump, in remarks to journalists on Sunday, warned that Iran had “better be careful” about its revival of the nuclear program.

Sanam Vakil, a researcher at Chatham House, in London, said, “It is hard to see an end game right now because it appears that the Trump administration is doubling down” but “the cost of that is going to be an increase in Iran’s escalatory reaction.”

“It looks like we are going to be in simmering conflict for the near term,” she said.

Britain, like other European powers, has been caught in the middle of the escalating feud. Britain, France, Germany and the European Union all signed the 2015 accord. Breaking with the Trump administration, all continue to support the deal, urging both the United States and Iran to resume compliance with its provisions.

As a group, the European powers have largely placed the blame for the breaching of the agreement primarily on the Trump administration, especially while Iran continued until recently to remain within its commitments under the deal.

But among the European powers, Britain has been the most wary of Iran and dubious about its intentions, diplomats say, making it a pivotal player in the deliberations over the future of the deal. If Britain decided to respond to Tehran’s steps over the limits of the deal by joining the United States in the resumption of sanctions, that could extinguish hopes of reviving the agreement.

Britain’s own clash with Tehran began last week when British forces seized the Iranian tanker off the coast of Gibraltar. British officials said they suspected the tanker of violating European Union sanctions on Syria; Iranian officials called the seizure an act of “piracy.”

If Britain did not release the tanker, Iran would “be duty-bound to take reciprocal action and seize a British oil tanker, Mohsen Rezaei, a senior officer in Iran’s elite Islamic Revolutionary Guards Corps warned last Friday.