BAGHDAD • Iran has vowed to hit back hard after a United States air strike in the early hours yesterday in Baghdad killed Major-General Qassem Soleimani, the commander of Iran's elite Quds Force and architect of its growing military influence in the Middle East.

Maj-Gen Soleimani was regarded as the second-most powerful figure in Iran after Supreme Leader Ayatollah Ali Khamenei.

The overnight attack, authorised by US President Donald Trump, marked a dramatic escalation in a "shadow war" in the Middle East between Iran and the US and its allies, principally Israel and Saudi Arabia.

US Secretary of State Mike Pompeo said the strike was in response to the threat of an "imminent attack".

He would not provide details about the threat other than to say it was confined to the Middle East and that US intelligence justified the air strike.

Top Iraqi militia commander Abu Mahdi al-Muhandis, an adviser to Maj-Gen Soleimani, was also killed in the attack.

Iranian state television reported that 10 people were killed in the air strike. Five of them were members of the Revolutionary Guards, including another general, a colonel, a major and a captain with the guard.

Iran has been locked in a long conflict with the US that escalated sharply last week with an attack on the US Embassy in Iraq by pro-Iranian militiamen following a US air raid on the Kataib Hizbollah militia, founded by Mr Muhandis.

Responding to the strike, Iraq's Prime Minister said Washington had violated a deal for keeping US troops in his country.

Israel put its army on high alert and US allies in Europe, including Britain, France and Germany, voiced concerns about an escalation in tensions.

China said it was "highly concerned" and called for all sides to exercise "calm and restraint".

The Pentagon said the "US military has taken decisive defensive action to protect US personnel abroad by killing Qassem Soleimani".

US officials, speaking on condition of anonymity, said Maj-Gen Soleimani was killed in a drone strike. Iran's Revolutionary Guards said he was killed in an attack by US helicopters.

Mr Pompeo tweeted that he had spoken with British Foreign Secretary Dominic Raab, German Foreign Minister Heiko Maas and Chinese Politburo member Yang Jiechi about the killing.

"Thankful that our allies recognise the continuing aggressive threats posed by the Iranian Quds Force," he wrote. "The US remains committed to de-escalation."

The US State Department issued a travel alert asking all US citizens to leave Iraq. "#Iraq: Due to heightened tensions in Iraq and the region, we urge US citizens to depart Iraq immediately. Due to Iranian-backed militia attacks at the US Embassy compound, all consular operations are suspended. US citizens should not approach the embassy," the State Department tweeted.

Ayatollah Khamenei called for three days of national mourning. He quickly appointed Maj-Gen Soleimani's deputy, Brigadier-General Esmail Ghaani, to replace him as the Quds Force head.

President Hassan Rouhani said the assassination would make Iran more decisive in resisting the US, while the Revolutionary Guards said anti-US forces would exact revenge across the Muslim world.

Tens of thousands of people took to the streets of Teheran to protest against American "crimes", chanting "Death to America" and holding up posters of the slain commander.

Mr Trump posted a picture of the US flag on Twitter after the strike became public. Later, the US President posted on the social media website: "Iran never won a war, but never lost a negotiation!"

In a separate tweet, he wrote, "General Qassem Soleimani has killed or badly wounded thousands of Americans over an extended period of time, and was plotting to kill many more... He should have been taken out many years ago!"

REUTERS, AGENCE FRANCE-PRESSE, NYTIMES, BLOOMBERG