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LONDON — European diplomats were hard at work Monday trying to ratchet down tensions between Iran and the United States after the U.S. killed a top Iranian military commander in Iraq last week, raising fears of all-out war in the Middle East.

But as President Donald Trump threatened to attack cultural sites in Iran if Tehran retaliates for the killing of Gen. Qassem Soleimani, some questioned whether a diplomatic solution to the crisis was still possible.

Iran vowed revenge after Soleimani was killed in a drone strike in Baghdad, and on Sunday announced that it was going to abandon limitations on enriching uranium that were negotiated under the 2015 nuclear deal, which the U.S. withdrew from in 2018.

Shortly after Iran’s announcement, the leaders of France, Britain and Germany called on Tehran to refrain "from further violent action" and reverse all measures inconsistent with the nuclear deal.

In a joint statement, German Chancellor Angela Merkel, French President Emmanuel Macron and British Prime Minister Boris Johnson called on all parties involved to exercise “utmost restraint and responsibility."

The three European powers also emphasized their readiness to talk to Iran directly.

“We stand ready to continue our engagement with all sides in order to contribute to defuse tensions and restore stability to the region,” the statement said. The trio were expected to discuss the situation on Monday.

Separately, E.U.’s foreign policy chief, Josep Borrell, invited Iranian Foreign Affairs Minister Javad Zarif to Brussels to discuss the crisis.

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German Foreign Minister Heiko Maas said in a statement Sunday that Europe had "reliable channels of communication" to all sides. "We will definitely talk to Iran again,” Maas told Deutschlandfunk radio Monday.

The European powers also seem to be in disagreement with Trump about his threat to impose sanctions on Iraq after its Parliament voted Sunday to end the U.S. military presence in the country.

Threatening Iraq with sanctions is "not very helpful," Maas said Monday. “I think the right way is to convince Iraq not with threats but with arguments."

Iran said it would continue to cooperate with the U.N. nuclear watchdog and could quickly reverse its latest steps if U.S. sanctions are removed.

“Iran still values the European efforts or at least they recognize the importance of having international sympathy and they have been pretty good at exploiting that sympathy since the U.S. withdrawal" from the 2015 nuclear deal, said Sanam Vakil, senior research fellow with Middle East & North Africa Programme at Chatham House, a think tank in London.

She said Iran's declaration on Sunday that it would abandon the limits set by the 2015 nuclear deal was a “relatively measured response designed to keep the door of diplomacy, discussion and engagement with Europe very open.”

Both Trump and the Iranians are very much backed into a corner in the wake of Soleimani's death, Vakil said, adding that she believed both sides were still looking for a diplomatic outlet.

“The Iranians have to respond to Soleimani’s killing. It would be very hard for them not to because of the symbolism of the way he was killed and his importance," she said. “Meanwhile, the U.S. president has not retreated or desisted from his statements, but continues to stand by them and double down when they are just inflammatory.”

“Having the international community lobby and pressure President Trump is an important part of Iran’s strategy right now,” Vakil added.

"In continental Europe, public opinion is clearly opposed to war, and no politician that seeks reelection would jeopardize their fortunes by beating the war drums against a country such as Iran,” Arshin Adib-Moghaddam, professor at the School for Oriental and African Studies in London, said. “So the Trump administration stands alone, which explains why none of the E.U. governments were consulted about [Soleimani's] killing.”

Other international powers have voiced their concerns about rising tensions.

Both Turkey and Saudi Arabia said via their foreign ministers Monday that they want a de-escalation of tensions while NATO ambassadors will be gathering in Brussels for an urgent meeting.

China has criticized the United States for aggravating tension in the Middle East and urged all parties to exercise restraint to ensure peace and stability. Japanese Prime Minister Shinzo Abe also called for peace, urging nations involved to make diplomatic efforts to ease tensions.

The Kremlin said Monday that Russian President Vladimir Putin will discuss the brewing crisis with Merkel when the two meet in Moscow this week. Germany and Russia are among the world powers that have been trying to salvage the 2015 nuclear deal.

Israeli Prime Minister Benjamin Netanyahu, one of the strongest supporters of Trump’s decision to target Soleimani, said the American president "deserves every appreciation" for acting with determination.

Netanyahu has pushed hard for tougher measures against Iran for years and advocated against the nuclear deal with Tehran.