The deaths of Gen. Qassem Soleimani and several associates drew immediate cries for revenge from Tehran and a chorus of appeals from other countries seeking reduced tensions between Iran and the United States. As U.S. Secretary of State Mike Pompeo called world capitals to defend the attack, diplomats tried to chart a way forward.

Iran's Supreme Leader Ayatollah Ali Khamenei warned that a "harsh retaliation is waiting" for the U.S. He moved quickly to appoint Soleimani's deputy, Maj. Gen. Esmail Ghaani, as the new commander of the Revolutionary Guard's Quds Force, which undertakes the country's foreign campaigns, including in Syria and Yemen.

U.N. Secretary-General Antonio Guterres urged leaders to "exercise maximum restraint," stressing in a statement that "the world cannot afford another war" in the Persian Gulf.

German Foreign Minister Heiko Maas echoed the U.N. chief saying, "A further escalation that sets the whole region on fire needs to be prevented." Maas also noted that the assault "followed a series of dangerous Iranian provocations."

In the United Arab Emirates, which sits across the Gulf from Iran, the minister of state for foreign affairs, Anwar Gargash, called in a tweet for rational engagement and a "calm approach, free of emotion." Qatar, which shares a massive underwater gas field with Iran, also called for restraint in a Foreign Ministry statement.

Saudi Arabia, Iran's top regional rival, added its own voice of caution against "all acts that may lead to aggravating the situation with unbearable consequences."

The White House sought to justify the killings with a tweet alleging that Soleimani "was actively developing plans to attack American diplomats and service members in Iraq and throughout the region."

"He should have been taken out many years ago!" U.S. President Donald Trump tweeted. But the president also told reporters: "We took action last night to stop a war. We did not take action to start a war."

Oil prices surged as investors fretted  about Mideast stability. Social media flooded with alarm. Twitter users morbidly turned "WWIII" into the top trending term worldwide.

"We are waking up in a more dangerous world. Military escalation is always dangerous," France's deputy minister for foreign affairs, Amelie de Montchalin, told RTL radio.

Russia's Foreign Minister Sergey Lavrov said Soleimani's killing "grossly violates international law and should be condemned."

He told Pompeo in a phone conversation late Friday that "the move by the U.S. is fraught with severe consequences for the peace of stability in the region and doesn't help resolve complicated problems in the Middle East," according to a ministry statement. Lavrov also urged Washington to "stop using unlawful methods of force" in trying to achieve its foreign policy goals and instead bring "any problems to the negotiating table.""

Foreign Ministry spokeswoman Maria Zakharova suggested earlier that Trump ordered it with one eye on his reelection campaign.

"The U.S. military were acting on orders of U.S. politicians. Everyone should remember and understand that U.S. politicians have their interests, considering that this year is an election year," Zakharova said in a TV interview.

Trump's election opponents characterized him as reckless. Democratic presidential candidate Joe Biden said the president "tossed a stick of dynamite into a tinderbox."

Iran's allies rallied to its side.

Syria's Foreign Ministry strongly condemned what it called "treacherous American criminal aggression." It said the attack reaffirmed U.S. responsibility for the instability in Iraq as part of its policy to "create tensions and fuel conflicts in the countries of the region."

The top leader of Yemen's Houthi rebels, who are Shiite and closely allied with Iran, offered his condolences to the Iranian people. Abdul-Malek al-Houthi, whose forces are fighting the internationally recognized government, said that Soleimani's "pure"' blood has not been shed in vain.

Afghan President Ashraf Ghani expressed concern in a statement that the death could mean more violence in the region. He said Afghanistan, despite the presence of 13,000 U.S. troops on its soil, does not want to be drawn into any confrontation between Washington and Tehran. Former Afghan President Hamed Karzai condemned the airstrike and said it violated international law.

U.S. allies Britain, Germany and Canada suggested that Iran bore some responsibility for the strike near Baghdad's airport. Iranian state TV said 10 people were killed.

German government spokeswoman Ulrike Demmer described the strike as "a reaction to a whole series of military provocations." She pointed to attacks on tankers and a Saudi oil facility, among other events.

"We are at a dangerous escalation point," she said.

The British foreign secretary, Dominic Raab, said his government had "always recognized the aggressive threat" posed by the Quds force.

Following Soleimani's death, "we urge all parties to de-escalate," he said. "Further conflict is in none of our interests."

Canadian Foreign Minister Francois-Philippe Champagne said Soleimani's "aggressive actions" had "a destabilizing effect in the region and beyond.

Brazil's President Jair Bolsonaro said in a televised interview:""I hope the moods calm down and that Iran modifies its way of conducting politics, as other Arab countries have done in the past."

There were warnings the killing could set back efforts to stamp out remnants of the Islamic State group.

A top European Union official, Charles Michel, said "the risk is a generalized flare-up of violence in the whole region and the rise of obscure forces of terrorism that thrive at times of religious and nationalist tensions."

Italy also warned that increased tensions "risk being fertile terrain for terrorism and violent extremism." But right-wing Italian opposition leader Matteo Salvini praised Trump for eliminating "one of the most dangerous and pitiless men in the world, an Islamic terrorist, an enemy of the West, of Israel, of rights and of freedoms."

Trump also won the support of Israeli Prime Minister Benjamin Netanyahu "for acting swiftly, forcefully and decisively."

In his calls to explain the strike to world leaders, Pompeo said the U.S. is committed to bringing down tensions that have soared since Iranian-backed militia killed an American contractor and the U.S. responded with strikes on the militia. That set off violent pro-Iran protests outside the U.S. Embassy in Baghdad, which in turn set the stage for the killing of Soleimani.

"Doing nothing in this region shows weakness. It emboldens Iran," Pompeo said. "We don't seek war with Iran, but we at the same time are not going to stand by and watch the Iranians escalate."

In the Mideast, the strike provoked waves of fury and fears of worse to come.

Iraq's most powerful Shiite religious leader, Grand Ayatollah Ali al-Sistani, said in a speech during Friday prayers that the country must brace for "very difficult times."

In Iran, a hard-line adviser to the country's supreme leader who led prayers in Tehran likened U.S. troops in Iraq to "insidious beasts" and said they should be swept from the region.

 "I am telling Americans, especially Trump, we will take a revenge that will change their daylight into to a nighttime darkness," said the cleric, Ayatollah Ahmad Khatami.