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Iran vowed to seek revenge on the United States on Friday after an American airstrike killed one of the country's most powerful military and political figures.

The U.S. bombing near Baghdad airport killed Qassem Soleimani, the high-profile commander of Iran's secretive Quds Force and sometimes described as the country's second most influential person.

"The great nation of Iran will take revenge for this heinous crime," Iranian President Hassan Rouhani tweeted after the news was confirmed.

The flag of General Soleimani in defense of the country's territorial integrity and the fight against terrorism and extremism in the region will be raised, and the path of resistance to US excesses will continue. The great nation of Iran will take revenge for this heinous crime. — Hassan Rouhani (@HassanRouhani) January 3, 2020

Iran's Supreme Leader Ayatollah Ali Khamenei said a "harsh retaliation is waiting for the criminals whose filthy hands spilled his blood." The country's foreign minister, Javad Zarif, called the U.S. strike "an act of state terrorism and a violation of Iraq's sovereignty."

Soleimani was one of the most influential figures in the Middle East, having developed a network of powerful militia groups whose clandestine reach stretched into Iraq, Syria and beyond.

His death is a major escalation in the recent tensions between Washington and Tehran, and Friday morning local time, the State Department urged all U.S. citizens "to depart Iraq immediately" because of the increased risk.

The strike was directed by President Donald Trump; House Speaker Nancy Pelosi, D-Calif., called on the administration to immediately brief lawmakers. She said the bombing was carried out without "authorization for use of military force" against Iran and without the consultation of Congress.

Multiple domestic U.S. law enforcement agencies said they were keeping a watchful eye on events overseas and ready to react as necessary.

Iran's supreme leader, Ayatollah Ali Khamenei, with the commander of the Iranian Revolutionary Guard's Quds Force, Gen. Qassem Suleimani, at a religious ceremony in Tehran. AFP - Getty Images file

The strike was met with caution by the United Kingdom, one of Washington's biggest allies that in the past has backed it in the Middle East and elsewhere.

"We have always recognized the aggressive threat posed by the Iranian Quds Force led by Qassem Soleimani," British Foreign Secretary Dominic Raab said. "Following his death, we urge all parties to de-escalate. Further conflict is in none of our interests."

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France's deputy minister for foreign affairs, Amelie de Montchalin, told her country's RTL radio station that the airstrike meant "we are waking up in a more dangerous world."

Secretary of State Mike Pompeo said in a series of tweets that he has been calling world leaders to reassure them that the "U.S. remains committed to de-escalation." But asked on CNN about the reaction from Paris, Pompeo said, "The French are just wrong about that."

Israeli Prime Minister Benjamin Netanyahu is cutting short his visit to Greece and was due back in Israel Friday.

"Just as Israel has the right of self-defense, the United States has exactly the same right," he said, adding that Trump "deserves all the credit for acting swiftly, forcefully and decisively."

In addition to being Iran's most senior general, Soleimani also held huge political clout, with many considering his influence second only to that of the supreme leader himself.

His Quds Force is a unit in Iran's powerful Revolutionary Guard and classified as a terrorist group by the U.S. and others.

The Iranian militias he cultivated are "responsible for the deaths of hundreds of American and coalition service members and the wounding of thousands more," according to the Department of Defense, which confirmed his death early Friday local time.

Hours after Soleimani's death, Iran announced that Brig. Gen. Esmail Ghaani would replace him as head of the Quds Force, according to the country's semi-official ISNA news agency.

Another man, Abu Mahdi al-Muhandis, said to be the deputy of the militias' umbrella group and a close adviser to Soleimani, was also killed in the U.S. airstrike, according to Iraqi television reports.

The deaths drew strong censure from Iraq's caretaker prime minister, Adel Abdul-Mahdi, who expressed his "utmost condemnation of the American administration’s assassination of the two martyrs" and called them "great symbols in achieving victory against the terrorist group ISIS."

The U.S. has around 5,000 troops in Iraq at the invitation of the government, their brief to assist government troops in the fight against the Islamic State militant group.

But the Iraqi government also recognizes and supports the militia groups, many of which are backed by Iran.

The country's prime minister called Soleimani's killing "a flagrant violation of the conditions for the presence of the American forces in Iraq,'' pointing out that "its role is limited to training Iraqi forces and fighting ISIS within the international coalition forces, under the supervision and approval of the Iraqi government."

Iraqi President Barham Salih told NBC News that "the situation in Iraq is very fragile, very precarious."

He said, "I urge restraint on all sides. Iraq cannot be condemned to another cycle of violence. We have had too many wars over the last four decades. The situation now is very dangerous, no doubt, and that is why I am urging calm."

The airstrikes also prompted some celebrations in Baghdad on Friday, with Pompeo tweeting a video that he said showed Iraqis "dancing in the street for freedom."

The flag of General Soleimani in defense of the country's territorial integrity and the fight against terrorism and extremism in the region will be raised, and the path of resistance to US excesses will continue. The great nation of Iran will take revenge for this heinous crime. — Hassan Rouhani (@HassanRouhani) January 3, 2020

In recent months, protests have erupted in the Iraqi capital. The demonstrators are angry at corruption and unemployment but also what they see as the complicity of neighboring Iran in their affairs.

Soleimani was a key figure in his country's involvement in Syria, where it was allied with Russia in supporting President Bashar al-Assad.

Russia's foreign ministry called the killing "an adventurist step" by the U.S. that would lead to increased tensions throughout the region, according to the Interfax news agency.

Meanwhile China urged "all parties concerned, especially the United States, to maintain calm and restraint and to avoid the further escalation of tension," foreign ministry spokesman Geng Shuang told a press briefing Friday.