Iran's Supreme Leader Ali Khamenei vowed "severe revenge" against the U.S. after the killing of the country's top security and intelligence commander Maj. Gen. Qassem Soleimani.

The leader said Soleimani's "efforts and path won't be stopped by his martyrdom, by God's power, rather a #severerevenge awaits the criminals who have stained their hands with his and the other martyrs' blood last night."

In his first public comments on the drone strike Thursday that killed at least six people, including deputy head of the Iranian-backed Popular Mobilization Forces Abu Mahdi al-Muhandis, President Donald Trump tweeted Friday morning that "Iran never won a war, but never lost a negotiation."

Secretary of State Mike Pompeo tweeted that he's spoken with government officials from several countries, including China, the United Kingdom and Germany, reiterating America's "commitment to deescalation."

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Russia's federal affairs chairman told state media that the killing of Soleimani "is very difficult news" and that it "looks very much like the revenge of the Americans." He warned that "retaliation … will not take long."

China urged the U.S. to "remain calm and exercise restraint to avoid further escalating tensions." The French embassy in Iran urged French citizens to stay away from public gatherings and to "display prudent and discreet behavior."

The U.K.'s Foreign Secretary Dominic Raab also advised "all parties to de escalate."

"Further conflict is in none of our interests," he added.

The State Department has told all Americans "to depart Iraq immediately" and said that all consular operations have been suspended due to Iranian-backed militia attacks at the U.S. embassy.

The department warned U.S. citizens to not approach the embassy in Baghdad.