BRUSSELS — The European Union’s chief diplomat took a defiant stance Tuesday after meeting with Iran’s foreign minister and other top European diplomats to try to salvage the Iran nuclear deal following President Trump’s decision to withdraw the United States.

Federica Mogherini, who negotiated the deal on behalf of the European Union, listed a string of proposals that taken together may not be enough to convince Iran’s leaders to hold to the deal but probably will be seen in Washington as a raised fist against U.S. policy.

The Trump administration has announced that it will be reimposing sanctions on Iran and is seeking to prevent companies around the world from doing business there.

Comparing the 2015 nuclear agreement to “a relative in intensive care,” Mogherini said ideas under consideration include plans to deepen Europe’s economic relationship with Iran, shield banking transactions with Tehran, keep purchasing Iranian oil and gas, and use E.U. financing for investments there.

“We are operating in a very difficult context,” Mogherini said after an intense day of diplomacy that included meetings with Iranian Foreign Minister Mohammad Javad Zarif and the foreign ministers of Britain, France and Germany. All of those countries were signatories of the deal that aimed to prevent Iran from building a nuclear weapon in exchange for economic incentives. She said European leaders hoped to have a firm plan within a few weeks.

Trump’s decision to pull out of the agreement unilaterally has created one of the biggest foreign policy crises of his presidency, infuriating Washington’s closest allies in Europe.

This week, European leaders will consider whether to order European companies doing business in Iran to ignore U.S. sanctions. That would be the diplomatic equivalent of a slap in the face.

And it may have limited effectiveness, since many companies would prefer access to the larger U.S. market over Iran’s relatively small one.

British Foreign Secretary Boris Johnson warned Tuesday that Washington’s sanctions may be able to strike around the world because of the long reach of the U.S. financial system.

“We have to be realistic about the electrified rail, the live wire of American extraterritoriality, and how that can serve as a deterrent to business,” he said.

Despite the E.U. efforts to save the nuclear agreement, Zarif was noncommittal on his way out of Brussels.

“It’s a good start. We’re not there. We’re beginning the process, and we need to receive those guarantees,” he said, striking a gloomier tone than he did at the outset of the day, when he appeared to be optimistic that the deal could be buttressed.

Trump’s decision has created a tricky game of strategy for other countries that remain committed to the agreement, which was endorsed by the U.N. Security Council after years of negotiations.

Iranian President Hassan Rouhani has staked his political legacy on delivering prosperity for Iranians through the deal, which is now threatened by the new U.S. sanctions. Iranian hard-liners who always felt the agreement made too many concessions may be emboldened to restart nuclear enrichment, but they could risk airstrikes from the United States and Israel if they did so.

The Iranian split was clear Tuesday, with some leaders in Tehran making threats even as Zarif indicated an openness to the talks in Brussels.

“We have the capacity and we are ready to resume our nuclear activities to a much higher level if the talks fail with Europeans to save the nuclear deal after America’s exit,” Iranian Atomic Energy Organization head Ali Akbar Salehi was quoted as saying by Iran’s Fars news agency, Reuters reported.

In Europe, leaders are faced with a decision about whether to treat their longtime closest ally, Washington, as an adversary, marshaling their economic might to engage in full-out economic conflict. They remain deeply dependent on the United States for security guarantees against Russia — something Trump has grumbled about — and are in the middle of negotiations over an exemption to new U.S. tariffs on steel and aluminum. Many also share U.S. concerns about Iranian behavior in the Middle East and its ballistic missile program, but they are divided on how to deal with the issue.

“We have experienced a break in German-American, in European-American relations,” German Chancellor Angela Merkel told a labor union congress in Berlin on Tuesday. “The agreement against nuclear weapons in Iran is an agreement that certainly has weaknesses, but an agreement that we should stand for.”