President Donald Trump said top Iranian general Qasem Soleimani was "terminated" when he was on the verge of attacking US diplomats but he insisted that Washington is not seeking to topple Iran's government.

"Soleimani was plotting imminent and sinister attacks on American diplomats and military personnel, but we caught him in the act," Trump said in Florida.

While referring to the key Iranian figure, killed in a US air attack earlier Friday in Iraq's capital Baghdad, as "sick," Trump attempted to lower tensions by insisting that he does not want war with Iran.

"We did not take action to start a war," he said, adding: "We do not seek regime change."

Iran, smarting from the loss of arguably the second most important leader in the country, erupted.

As head of Iran's Revolutionary Guard Corps' foreign operations arm, Soleimani was a powerful figure domestically and pointman for sophisticated and wide-ranging Iranian involvement in regional power struggles -- and anti-US forces.

Iran's supreme leader Ayatollah Ali Khamenei swiftly promised "severe revenge" and tens of thousands of protesters in Tehran torched US flags and chanted "death to America."

US officials said Suleimani, 62, was killed by a missile fired from a drone when he was near Baghdad's international airport.

Five Revolutionary Guards were killed, along with five members of a pro-Iranian paramilitary force in Iraq, the Hashed al-Shaabi, including their deputy chief.

Iran named a replacement for Soleimani -- his previous deputy, Esmail Qaani.

- Embassy storming -

Soleimani had long been considered a lethal foe by US lawmakers and presidents. The last straw was an attack by a pro-Iranian mob in Baghdad against the US embassy this week.

Demonstrators managed to burn part of the compound, besieging diplomats inside.

Trump said Soleimani should have been killed "many years ago."

But previous presidents have held back from what they feared would be an overly provocative step against Iran, which has been in tense confrontation with the United States and its main regional ally Israel for decades.

For Trump, who is campaigning for reelection on a platform of getting out of wars, the situation is high risk.

The US embassy in Baghdad urged all American citizens to leave Iraq immediately and US nationals working at southern oil fields were being evacuated, Iraq's oil ministry said.

And having repeatedly claimed to be pulling US troops out of conflict zones, Trump sent more soldiers into the Middle East.

A Pentagon official said 3,000 to 3,500 troops from the 82nd Airborne Division's Global Response Force, which already had sent hundreds of reinforcements earlier this week, will go to Kuwait.

Some 14,000 other troops have already been deployed as reinforcements to the Middle East this year, reflecting steadily growing tensions with Iran.

Signalling growing threats to the approximately 5,200 US troops still in Iraq, a leading Hashed member, Hadi al-Ameri, urged lawmakers there "to oust foreign troops... because their presence has become a threat."

- Iraq criticizes, Israel praises -

Iran's Foreign Minister Mohammad Javad Zarif slammed the US strike as "extremely dangerous and a foolish escalation," as Khamenei declared three days of mourning.

Iraqi Prime Minister Adel Abdel Mahdi called the strike a "flagrant violation" of a security accord with the US, warning it would "spark a devastating war in Iraq."

Paramilitary figures in Iraq including US-blacklisted Qais al-Khazali and militiaman-turned-politician Moqtada Sadr called on their fighters to "be ready."

And in Lebanon, the leader of the Tehran-backed Shiite movement Hezbollah, Hassan Nasrallah, warned of "punishment for these criminal assassins."

But there were daring celebrations in Baghdad's Tahrir Square, epicenter of a three-month-old protest movement against government corruption and Iranian influence.

Israeli Prime Minister Benjamin Netanyahu praised Trump "for acting swiftly, forcefully and decisively," as the Jewish state braced itself for possible reprisal attacks.

- 'Decapitation strike' -

Analysts said the strike -- which sent world oil prices soaring -- would be a game-changer in the tensions between Iran and the US.

"Trump changed the rules -- he wanted (Soleimani) eliminated," said Ramzy Mardini, a researcher at the US Institute of Peace.

Phillip Smyth, a US-based specialist on Shiite armed groups, described the killing as "the most major decapitation strike that the US has ever pulled off."

He told AFP it would have "bigger" ramifications than the 2011 US operation that killed Al-Qaeda chief Osama bin Laden and the 2019 American raid that killed Islamic State group Abu Bakr al-Baghdadi.

Ties between the US and Iran have deteriorated markedly since Washington abandoned a landmark nuclear deal with Tehran in 2018 and reimposed crippling sanctions.