BRUSSELS — European diplomats who gathered Monday to try to save the Iran nuclear deal warned that the agreement was close to unraveling, but several held out hope they would be able to lure Tehran back into compliance.

“We think there is a closing but small window to keep the deal alive,” British Foreign Secretary Jeremy Hunt said on his way into a meeting of European Union foreign ministers in Brussels. “What we’re looking for is to give Iran a way out of this.”

Britain, France and Germany — all signatories of the 2015 deal — have been reluctant to reimpose sanctions on Iran, despite Tehran’s decision to breach uranium-enrichment limits and in defiance of calls from Washington to join a “maximum-pressure” campaign.

The Europeans maintain that Tehran’s recent moves reflect a desire to win more economic benefits rather than to abandon the agreement. European leaders also remain furious at President Trump for unilaterally walking away from a deal they see as essential to avoid an Iran with nuclear weapons.

None of the parties to the deal believes that Iran is in “significant noncompliance,” E.U. foreign policy chief Federica Mogherini said Monday after the meeting here.

She said that the steps taken so far by Iran were reversible and that European leaders had no intention at this point of triggering the reimposition of sanctions or the unraveling of the agreement.

The deal is “not in the best of health, but it’s still alive,” Mogherini said. “We’re at the most difficult juncture in its history, but the agreement is still in place.”

The Europeans hope to convince Iranians that the deal is still worthwhile through the launch of a complicated trading tool designed to shelter certain transactions from U.S. sanctions. Mogherini said the tool, known as Instex, was now operational and processing its first transactions — though she admitted Monday that “we definitely expected it to be faster.”

She said leaders were still considering whether to allow Iranian oil to be sold through the tool, a step that would more directly defy Washington but could channel more money to Iran.

European diplomats, however, also condemned Iran for departing from the rules of the agreement.

The Iranian decision to breach the deal was “a bad response to a bad decision” by the U.S. to impose sanctions, French Foreign Minister Jean-Yves Le Drian told reporters on his way into the Brussels meeting Monday.

European leaders warned that pressures from Washington and Tehran could stretch the deal to its breaking point. 

Tensions have spiked following a spate of attacks on commercial tankers and the downing of a U.S. Navy spy drone by Iranian forces last month. Britain this month detained an Iranian oil tanker that it said was carrying fuel to Syria in violation of sanctions. Iranian ships last week then tried to block a British oil tanker traversing the Strait of Hormuz just off Iran’s coast, but they were turned back by a British naval escort.

Adding to the strains, France’s Foreign Ministry on Monday said that a French Iranian researcher, Fariba Adelkhah, who works at Paris’s prestigious Sciences Po, had been detained in Iran, and French officials had been unable to gain access to her.

Iran offered no immediate confirmation of the detention.

Iran’s foreign minister, Mohammad Javad Zarif, said Sunday that Europe needs to do more to persuade Tehran to adhere to the nuclear deal. He spoke to reporters in New York, where he was attending a meeting of the United Nations Economic and Social Council on sustainable development goals.

Pledging commitment to the agreement “is totally different from being ready to make the investments required to save the deal,” Zarif said, according to Iran’s Tasnim News Agency.

“We have yet to see that [the Europeans] are prepared to invest in Iran,” he said.

Some Europeans say they have little wiggle room in their effort to save the deal, which is formally called the Joint Comprehensive Plan of Action, or JCPOA.

“We are being accused at the same time of supporting the Iranian regime’s violations of the JCPOA and of being too weak against the U.S.,” said a senior E.U. diplomat, speaking on the condition of anonymity to discuss sensitive calculations.

China, which is also a party to the agreement, has called on the United States to reverse course, with a Foreign Ministry spokesman on Monday blaming Washington for the current tensions.

Under the terms of the deal, any of the signatories could trigger a dispute process if they believed Iran was in serious breach. That would give about a month for negotiations before sanctions might be reimposed. But the Europeans say they are not yet at a point where they want to start the clock. 

Iran is still allowing international inspectors to keep close watch over its uranium enrichment, the diplomat said. And the two steps Iran has taken that break the rules of the agreement — modestly increasing its stockpile of low-enriched uranium and the level of enrichment — do not take it significantly closer to producing a nuclear weapon.

If Iran ramps up its enrichment, kicks out inspectors or puts mothballed centrifuges back into operation to bolster its nuclear program, that could change Europe’s calculation, the diplomat said.

Europe is also worried about an upcoming early-August deadline for Washington to extend sanctions waivers on efforts to overhaul Iranian nuclear facilities to reduce their ability to make nuclear fuel. The exemptions allow Europe, Russia and China to contribute to the work. U.S. officials have discussed ending the waivers to increase pressure on Tehran. But many Europeans fear that if the waiver ended, Iran would break more decisively from the agreement, leading to its unraveling.

Erin Cunningham in Dubai contributed to this report.