BERLIN — European leaders on Friday warned that the targeted killing of Iran's top military commander by the United States could unleash an unpredictable blowback, putting allied troops at risk, straining already troubled transatlantic ties and dealing a death blow to the Iran nuclear deal.

German Foreign Minister Heiko Maas said that the Trump administration's order to kill Maj. Gen. Qasem Soleimani, the head of Iran's elite Quds Force, had come after "a series of dangerous provocations by Iran" but had "not made it easier to reduce tensions."

Maas said he had expressed his concerns "clearly" to Secretary of State Mike Pompeo, who called his British and German counterparts on Friday.

For those in Europe who may have wished to ride out recent tensions with the United States and put a Band-Aid on world problems for the duration of the Trump era, their hopes appeared to dim.

Several European diplomats said Friday that they were not aware of any warning from Washington ahead of the strike on Soleimani in Baghdad, though the mission was almost certain to increase the security risk for hundreds of European troops and for other European citizens in the region.

The Pentagon described the killing as a “defensive action.” Iran vowed “severe revenge.”

Nathalie Tocci, the director of the Rome-based Italian International Affairs Institute, said the strike against Soleimani was “irresponsible madness” that was likely to expose Europeans in the Middle East, particularly in Iraq, where Iran might aim its counterstrikes. Tocci said the fear was that American troops would have to abandon Iraq, leaving its allies exposed.

“Europe was always subservient to U.S. policy and interests, and it’s no different now,” she said. “What is different now is that U.S. policy seems to be even more reckless than it was in 2003.”

In the immediate aftermath of the strike, the State Department urged U.S. citizens to leave Iraq. The Netherlands issued similar guidance to its citizens. The French Embassy in Tehran told people to avoid demonstrations and stay inconspicuous.

Christina Routsi, a spokeswoman for the German Defense Ministry, said 130 German military personnel who had been training Iraqi forces had been confined to their bases in Taji and Baghdad.

Italy’s Defense Ministry raised the security level at locations where its soldiers operate overseas and said it was limiting movements outside of bases to a “minimum.”

A chorus of European leaders on Friday urged a defusing of tensions between the United States and Iran.

Maas said that further escalation, “which could set the whole region on fire, must be prevented.” British Foreign Secretary Dominic Raab said Britain had “always recognized the aggressive threat posed” by Soleimani, but “further conflict is in none of our interests.” The Italian Foreign Ministry called for “moderation and responsibility.”

European officials generally avoided criticizing the U.S. move. They blamed Iran for provocative actions such as attacks on tankers in the Strait of Hormuz and on Saudi oil fields.

Jürgen Hardt, foreign policy spokesman for German Chancellor Angela Merkel’s Christian Democrats, said Iran had “systematically expanded its destabilizing activities in the Middle East in recent years” and “exceeded a new escalation threshold” by backing a violent protest at the U.S. Embassy in Baghdad this week.

However, Hardt said it was “doubtful” that targeting Soleimani was well advised, as it is unlikely to weaken Iran and could unleash a “new wave of violence.”

Donald Tusk, who was president of the European Council until November, was more blunt about the U.S. role, tweeting that “President Trump’s decisions provoke global risks and his intentions remain unclear.” Still, Tusk urged Europe and the United States to “maintain transatlantic unity in the face of the approaching political earthquake,” seemingly a reference to the Iranian reaction to Soleimani’s death.

European leaders said they feared the strike could mark the final end to their struggling efforts to preserve the Iran nuclear deal.

In 2018, Trump pulled the United States out of the agreement, also known by its initialism, JCPOA.

The Europeans stuck with it. They see restraining Iran’s nuclear enrichment as central to their own security, though they have struggled in their efforts to keep Tehran engaged and though, even before the strike, Iran was considering new steps to breach the deal.

“I cannot imagine how the JCPOA can still be relevant,” said one senior E.U. diplomat, speaking on the condition of anonymity to discuss European concerns about the fallout from the strike. “There will be strong turbulence regionally and globally.”

The diplomat said one fear is that Iran could demand E.U. support against U.S. actions and then walk away from the deal when European backing failed to materialize. That diplomat and others said that ultimately, most European nations remain aligned with the U.S. view that Iran is a dangerous actor, and that if forced to choose between one side or the other, they would ultimately pick Washington.

But the diplomat said splits could open inside Europe about how firmly to support the United States.

Hardt said Germany would continue to back the Iran nuclear deal, as there “isn’t anything else on the table.” The U.S. policy of “maximum pressure” through sanctions cannot succeed without the support of Russia and China, he said.

Others said that was wishful thinking.

“The JCPOA has been dead for weeks, but nobody can publicly admit it,” said Markus Kaim, a security expert with the German Marshall Fund of the United States. The strike on Soleimani leaves “no common ground for a common transatlantic approach” to containing Iran, he said.

Iran was already expected to be days away from announcing fresh reductions in its compliance with the nuclear deal, and Europeans had been growing deeply frustrated by Tehran’s violations.

“We could well end up in a scenario where the Iranians not only increase enrichment, but may restrict the access to inspectors,” said Tocci, who was an adviser to the E.U. officials who negotiated the deal. She said Iran may pull out of the Nuclear Non-Proliferation Treaty altogether.

If Iran decides to use the moment to escalate its nuclear enrichment, E.U. leaders will probably feel compelled to trigger a formal mechanism in the agreement that would lead to the reimposition of European sanctions against Iran.

Major additional Iranian violations of the deal “would push the Europeans, whether they like it or not, towards a harder stance on Iran, which might ultimately lead to more alignment with Washington,” said Luigi Scazzieri, a research fellow at the Center for European Reform, a London-based think tank.

But don’t expect a convergence of U.S. and European strategies on Iran, Scazzieri said.

Trump has a sharply different view of foreign policy from most European leaders. And the Europeans will probably have to give up the idea that they can shape events in the Middle East in a way that would help them safely ride out Trump’s time in office, he said.

Birnbaum reported from Brussels. Chico Harlan and Stefano Pitrelli in Rome, William Booth in London, and Luisa Beck in Berlin contributed to this report.