SOFIA, Bulgaria — Venting anger at President Trump, European leaders said Thursday they would take steps aimed at blunting the effects of the American sanctions he restored on Iran, which could penalize European companies doing business there.
At a European Union summit meeting in Sofia, Jean-Claude Juncker, the president of the European Commission, the bloc’s executive arm, said it would begin a legal process to prohibit companies based in the 28-nation union from complying with the American sanctions.
“We should know that the effects of the announced American sanctions will not remain without consequences,” Mr. Juncker said. “We have a duty to protect our European companies.”
The meeting, with leaders of western Balkan countries, came barely a week after Mr. Trump quit the 2015 nuclear agreement between Iran and leading world powers, which has eased sanctions on the country in exchange for verifiable curbs on its nuclear activities.
Leaders of Britain, France and Germany had implored Mr. Trump in vain to honor the accord, which he has long described as a disastrous giveaway.
The other parties to the agreement, including Iran, China and Russia, have said they want the agreement to survive. At the same time, Iran has hinted that it may no longer honor the agreement if the promised economic benefits do not begin flowing to the nation of 80 million.
The withdrawal of the United States is a potentially fatal blow to the agreement because it restores all of the American sanctions, which not only restrict commerce with Iranian energy, banking and other sectors, but could penalize foreign businesses that trade with or invest in Iran.
On Tuesday the Trump administration took further steps to isolate Iran economically, placing the governor of its central bank on a terrorist blacklist.
The American sanctions are a powerful deterrent to many non-American companies, particularly large multinational businesses that are intertwined in the United States economy and dependent on its financial system. The restored sanctions mean they must essentially choose between doing business with Iran or the United States.
Mr. Trump’s Iran decision is part of a broader pattern of actions that have deeply frustrated America’s European allies. He also has threatened new tariffs in trade negotiations, quit the Paris climate accord and, in their view, expressed disdain for multilateral diplomacy.
Their anti-Trump mood was reflected in a speech on Wednesday by Donald Tusk, president of the European Council, who denounced what he called the “capricious assertiveness of the American administration.”
Mr. Juncker amplified the criticism on Thursday.
“We will not negotiate with the sword of Damocles hanging over our heads,” Mr. Juncker said. “It’s a matter of dignity, and it’s a matter of principle.”
Prime Minister Charles Michel of Belgium said Europeans were united in preventing the United States from determining the European Union’s economic interests.
Dalia Grybauskaite, the president of Lithuania, noted that Europe does not want a trade war with the United States over the Iran deal. “We are looking for technical solutions to protect our companies,” Ms. Grybauskaite said.
Despite their defiant response to Mr. Trump, it is unclear how effective any European Union measures would be in countering the restored American sanctions on Iran. Even as the summit meeting was underway, more big European-based companies signaled they would have to quit doing business with Iran to avoid running afoul of the American sanctions.
The world’s leading container shipping company, A.P. Moller-Maersk, said it would no longer do business in Iran, Reuters reported, while the Italian steel maker Danieli began scaling back on Iranian orders. On Wednesday, the French oil giant Total said it would have to divest from Iran unless it received an exemption from the American sanctions.
President Emmanuel Macron of France appeared to acknowledge this reality at a news conference in Sofia, conceding that France does not want to get into a trade war with the United States over Iran.
“We’re not going to choose one camp over another. We’re not going to be the allies of Iran against the United States of America,” Mr. Macron said. While he said he wants to protect the ability of European businesses to remain in Iran if they wish, Mr. Macron also said, “We’re not going to impose on French businesses to stay in Iran.”
Reporting was contributed by Milan Schreuer from Brussels; Boryana Dzhambazova from Sofia, Bulgaria; and Rick Gladstone from New York.