The United States has killed the powerful commander of Iran's elite Quds Force in an Iraqi air strike in a dramatic escalation of hostilities that prompted a threat from Tehran of "severe retaliation" and an accusation by the Iraqi prime minister of an "outrageous breach."

U.S. and Iranian officials confirmed the death of Qasem Soleimani, one of the most powerful military men in Iran, in an attack on two vehicles at Baghdad’s international airport in the early morning hours of January 3.

U.S. President Donald Trump tweeted later that Soleimani had "killed or badly wounded thousands of Americans over an extended period of time, and was plotting to kill many more...but got caught!" He blamed him "directly and indirectly" for the deaths of "millions of people, including the recent large number... of protesters killed in Iran itself."

U.S. Secretary of State Mike Pompeo told Fox News later on January 3 that the air strike was "lawful" and that "the risk of doing nothing regarding Iran was enormous." The United States does not "seek war with Iran," he said, but will not "stand by and see American lives put at risk."

The United States has "done all it can to fortify American assets in the region," Pompeo added.

He said the air strike on Soleimani disrupted an "imminent attack," adding that "there is no doubt" that it saved American lives.

Asked about threats within the United States, Pompeo told CNN that the threats were "in the Middle East."

He also tweeted that "the U.S. remains committed to de-escalation."

The assassination of Soleimani follows days of increased tensions since the United States struck an Iran-backed militia in Iraq and Syria that Washington blamed for repeated attacks on Iraqi bases that house U.S. troops. A mob that included pro-Iran paramilitary groups attacked the U.S. Embassy after those U.S. bombings before withdrawing on January 1.

Iranian Supreme Leader Ayatollah Ali Khamenei threatened “severe retaliation” against the “criminals” who killed Soleimani, whose public profile had risen over the past decade as Tehran fought alongside Syrian and Iraqi troops to beat back anti-government forces in both those countries.

Khamenei declared three days of national mourning.

State television said that the U.S. strike had killed 10 people, five of whom it said were members of the Islamic Revolutionary Guards Corps (IRGC), which helps oversee the Quds Force. That number could not be independently confirmed.

In Tehran, the normal Friday Prayer crowd swelled as tens of thousands of Iranians came out to express defiance toward the U.S. and its allies.

Video and still images on social media showed crowds in a number of cities.

Meanwhile, Khamenei named a longtime Quds deputy head, Ismail Qaani, as the force's new commander.

Qaani was the source of a 2012 interview that was later scrubbed from an official news agency's website in which he touted Iran's presence in Syria -- "physically and nonphysically" -- as preventing "big massacres."

Sources from the Shi’ite-led Hashd Shaabi militia (Popular Mobilization Forces, or PMF), which is backed by Iran, said the strike also killed Abu Mahdi al-Muhandis, the deputy commander of the PMF.

Iran's Islamic Revolutionary Guards Corps (IRGC) confirmed Muhandis' death in a statement.

The Iraqi military said in a statement that the targeting of Muhandis was "a clear breach of [the] U.S. troop mandate" in Iraq, Reuters said.

In an apparent reference to Soleimani and Muhandis, Iraqi caretaker Prime Minister Adil Abdul-Mahdi said "the two martyrs were huge symbols of the victory against Islamic State."

Abdul-Mahdi, whose government hinted earlier this week that it could consider changes to its arrangements concerning the U.S. presence in the country, called the strike that killed Soleimani "an outrageous breach of the conditions for the presence of U.S. forces in Iraq," according to Reuters.

He called it a "dangerous escalation that will light a fuse of a destructive war in Iraq, the region, and the world," and invited the Iraqi parliament to convene an extraordinary session and said it should take decisions to ensure Iraq's "dignity, security, and sovereignty," according to Reuters.

Iraqi President Barham Salih urged unity to protect the national interest and security.

He said the country "must avoid the tragedies of armed conflict that have plagued it over four decades," Reuters said.

The U.S. Embassy reportedly told U.S. citizens to "depart Iraq immediately" after news of the strike.

The United States will send nearly 3,000 more Army troops to the Mideast as reinforcements, U.S. defense officials said later on January 3.

Oil prices soared 4 percent immediately following reports of the attack.

The U.S. military confirmed the strike shortly after President Donald Trump tweeted out a U.S. flag in his first Twitter posting in almost 13 hours.

Later on January 3, Trump tweeted that "Iran never won a war, but never lost a negotiation!"

The Pentagon said Trump had approved the attack on the morning of January 2.

"At the direction of the President, the U.S. military has taken decisive defensive action to protect U.S. personnel abroad by killing Qasem Soleimani, the head of the Iranian Revolutionary Guard Corps-Quds Force, a U.S.-designated Foreign Terrorist Organization," the U.S. military said in a statement.

"This strike was aimed at deterring future Iranian attack plans," it added.

The statement said Soleimani had organized attacks on U.S.-led coalition bases in Iraq over the past several months, including one that killed a U.S. contractor on December 27.

"General Soleimani also approved the attacks on the U.S. Embassy in Baghdad that took place this week," it said.

AFP quoted an unnamed U.S. defense official as saying the attack had been carried out by a "precision drone strike [that] hit two vehicles at Baghdad airport.

Pompeo said the United States could only confirm the death of Soleimani.

Pompeo later called Afghan President Ashraf Ghani to discuss Trump's decision, according to U.S. State Department spokesperson Morgan Ortagus.

Soleimani headed the Quds Force, the foreign arm of Iran's IRGC. The force has been designated as a foreign terrorist organization (FTO) by the United States.

Iranian President Hassan Rohani said in televised remarks that the assassination will make Tehran more decisive in its resistance against the United States.

"Soleimani's martyrdom will make Iran more decisive to resist America's expansionism and to defend our Islamic values. With no doubt, Iran and other freedom-seeking countries in the region will take his revenge," Rohani said, according to Reuters.

Iranian media said the Foreign Ministry had summoned the Swiss charge d'affaires, whose country represents U.S. interests in Iran in the absence of formal diplomatic relations between Washington and Iran, and informed the envoy of its "strong protest."

'Dangerous Escalation'

In Damascus, the Foreign Ministry of Iranian ally Syrian President Bashar al-Assad's administration called the U.S. attack "cowardly" and "treacherous, criminal American aggression."

Beijing urged "calm and restraint" after the news of the attack, and said "peace and stability must be upheld," according to Reuters. China urged "all relevant sides, especially the U.S., to remain calm and exercise restraint," the agency said.

Moscow warned that the assassination will "increase tensions."

While U.S. Republicans hailed Trump's decision, many Democrats criticized the attack, saying it would put U.S. personnel in danger.

House Speaker Nancy Pelosi (Democrat-California) called it a "dangerous escalation," according to AFP.

Joe Biden, a leading Democratic challenger to Trump in the 2020 election, said that "no American will mourn" the passing of the Quds Force leader.

But he added that "President Trump just tossed a stick of dynamite into a tinderbox, and he owes the American people an explanation of the strategy and plan to keep safe our troops and embassy personnel."

Iran's semiofficial Fars news agency quoted a spokesman for the country's top security body as saying its members would meet to discuss the "criminal attack" on Soleimani and the others.

'Rogue Adventurism'

Iranian Foreign Minister Mohammad Javad Zarif wrote in a tweet that the U.S. "act of international terrorism, targeting & assassinating General Soleimani --THE most effective force fighting Daesh [Islamic State], Al Nusrah, Al Qaeda et al -- is extremely dangerous & a foolish escalation. The U.S. bears responsibility for all consequences of its rogue adventurism."

Unconfirmed reports said at least one member of Lebanon's Hizballah movement was also killed in the attack.

Earlier Iraqi paramilitary groups backed by Iran said that five members of their groups and two "important guests" were killed in an air strike on their vehicles inside the territory of Baghdad International Airport.

The Al Arabiya broadcaster had reported that an official with the PMF had been killed in the attack in the early morning hours, identifying him as Mohammed al-Jaberi, head of public relations for the militia.

In July 2018, Soleimani said his forces were ready to confront the U.S. military should Trump act on his warning that Tehran will "suffer consequences" if it threatens the United States.

"Mr. Trump, how dare you threaten us?" Soleimani was quoted as saying at the time.

The reports of the attack come during a period of raised tensions between Washington and Tehran over actions in Iraq.

On December 31, thousands of supporters of the Shi’ite PMU militia broke into the U.S. Embassy compound in central Baghdad.

The embassy attackers said they were protesting recent U.S. air strikes that killed at least 25 members of an Iran-backed militant group.

On January 2, Iranian military leaders warned Washington against threatening military action after Trump said Tehran would be held responsible for recent anti-U.S. protests in Iraq, including the embassy siege.

"We are not leading the country to war, but we are not afraid of any war and we tell America to speak correctly with the Iranian nation,” Brigadier General Hossein Salami, the head of the Islamic Revolutionary Guards Corps (IRGC), said on January 2.

“We have the power to break them several times over and are not worried," he said in a speech in the southwestern Iranian city of Ahwaz.

Meanwhile, army chief Major General Abdolrahim Musavi said Iranian armed forces were ready to confront the "enemy."

Prior to reports of the air strikes in Baghdad, U.S. Defense Secretary Mark Esper said Iran or its proxy forces may be planning further strikes on American interests in the Middle East, adding that the United States would take action -- preemptively, if it had sufficient warning.

With reporting by AFP, dpa, Reuters, AP, Al-Jazeera, RFE/RL's Radio Farda, and RIA Novosti