US President Donald Trump ordered the killing of Iran Revolutionary Guards commander Qasem Soleimani, who died in Baghdad "in a decisive defensive action to protect US personnel abroad," the Pentagon said.

"General Soleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region. General Soleimani and his Quds Force were responsible for the deaths of hundreds of American and coalition service members and the wounding of thousands more," the Department of Defence said.

Following Soleimani's death, Trump tweeted an image of the US flag without any further explanation.

The strike, which occurred at Baghdad's international airport on Friday in Iraq, also killed the deputy chief of Iraq's powerful Hashed al-Shaabi paramilitary force.

A pro-Iran mob this week laid siege to the US embassy following deadly American air strikes on a hardline Hashed faction.

The US had called the strikes in response to a rocket attack days earlier that had killed an American contractor working in Iraq.

The Baghdad airport was hit in a volley of missiles just after midnight Friday, Iraq's military had announced.

Security sources told AFP the bombardment hit a Hashed convoy and killed eight people, including "important figures." Soleimani heads the Islamic Revolutionary Guard Corps' Quds Force and also serves as Iran's pointman on Iraq, visiting the country in times of turmoil.

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

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

The Pentagon said that Soleimani had orchestrated attacks on coalition bases in Iraq over the past months, including on December 27, the day the US contractor was killed.

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