LONDON — Britain, France, and Germany have joined forces to urge the United States to exempt European companies from any sanctions the United States will slap on Iran after pulling out of an international nuclear agreement.

In a letter dated Monday to US Treasury Secretary Steven Mnuchin and Secretary of State Mike Pompeo, ministers from the three European countries said they ‘‘strongly regret’’ last month’s decision by President Trump to withdraw from the Iran deal. Trump, who argued the 2015 agreement was insufficiently tough on Iran, has said sanctions will be imposed on any company doing business there. The three European countries were also signatories of the 2015 deal, meant to stop Iran from developing nuclear weapons in exchange for the lifting of economic sanctions.

The ministers said ‘‘as close allies we expect that the extraterritorial effects of US secondary sanctions will not be enforced on EU entities and individuals, and the United States will thus respect our political decision and the good faith of economic operators within EU legal territory.’’

The ministers, who included British Foreign Minister Boris Johnson, French Finance Minister Bruno Le Maire, and his German counterpart, Olaf Scholz, want the United States to ‘‘grant exemptions’’ for EU companies that have been doing business with Iran since the deal took effect in 2016. They also said Iran should not be cut out of the SWIFT system for international money transfers.

Many companies from Europe and the United States have been steadily building up their investments in Iran in the past few years, particularly in the fields of pharmaceuticals, banking, and oil. Any sanctions could be damaging, especially if they affect their business interests in the United States.

The ministers reiterated their view that the deal with Iran remains the ‘‘best means’’ to prevent the country from becoming a nuclear power. ‘‘There appears to be no credible alternatives at this time,’’ they said.

They also warned that any Iranian withdrawal from the deal would ‘‘further unsettle a region where additional conflicts would be disastrous.’’

Publication of the letter came a day after Iran said it’s preparing to resume uranium enrichment within limits set by the 2015 agreement. The modest steps appeared to be mainly aimed at signaling that Iran could resume its drive toward industrial-scale enrichment if the nuclear accord unravels.