Iraqi paramilitary chief Abu Mahdi al-Muhandis, killed early on Friday in a US air strike in Baghdad, was seen as Tehran's man in Iraq and a sworn enemy of the United States.

The US strike on Baghdad's international airport also killed Iranian general Qassem Soleimani, the head of Iran's Islamic Revolutionary Guard Corps' (IRGC) Quds Force and a friend of al-Muhandis.

The men died in an air strike on a convoy belonging to Hashd al-Shaabi (Popular Mobilisation Forces) an Iraqi paramilitary force with close ties to Iran, whose deputy chief was al-Muhandis.

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It came just days after Hashd members and supporters attempted to storm the US embassy in Baghdad after the US launched air attacks against Kataib Hezbollah positions in Iraq and Syria.

Al-Muhandis - the widely-used nom-de-guerre for Jamal Jaafar al-Ibrahimi - was among the crowds of PMF members and supporters that protested at the embassy on Tuesday.

Known for his strong anti-US rhetoric during the US-led occupation of Iraq, al-Muhandis, 56, also built up close ties to Iran over decades.

"Muhandis was demonstrative of how Iran built its network of proxies in Iraq," said Phillip Smyth, a US-based researcher focused on Shia armed groups, as cited by AFP news agency.

"He has history with basically every major network Iran had in Iraq. You would not have found a stronger ideal" of Iran's influence in the country, he said.

Born in 1953 in Iraq's southern Basra, al-Muhandis held both Iraqi and Iranian citizenship.

He started his political life with the Dawa party, a Shia group that was crushed by Iraqi leader Saddam Hussein in the 1970s. Like others in the party, including the future Prime Minister Nouri al-Maliki, al-Muhandis fled abroad and joined forces with Iran.

In the 1980s, he became commander of the Badr Corps, a unit of Iraqi fighters founded in Iran in opposition to then-president Hussein.

He was sentenced to death in Kuwait for involvement in the 1983 bomb attacks on US and French embassies there but fled the country.

'Inveterate opponent of US'

After Hussein's removal in the US-led invasion in 2003, al-Muhandis briefly served as a member of parliament in Iraq following the 2005 elections.

He then helped found Kataib Hezbollah - an Iran-backed paramilitary group that has targeted US troops. The US targeted three Kataib positions in Iraq and two in Syria on Sunday in response to a rocket attack in northern Iraq last week that the US blamed on the group - and which killed a US contractor.

In 2009, the US sanctioned both al-Muhandis and Kataib Hezbollah as "terrorist" entities.

Washington said he ran "weapons smuggling networks and participated in bombings of Western embassies and attempted assassinations in the region."

Michael Knights, an expert at the Washington Institute, described al-Muhandis as "the most inveterate opponent of the United States" among Iraq's Shia armed groups.

He was later appointed deputy head of the Hashd, founded as a loose network of Shia-majority factions fighting the ISIL (ISIS) group in Iraq.

It was later absorbed into Iraq's formal security forces, but critics say that some of the factions, including Kataib Hezbollah, still operate independently of Baghdad.

"Muhandis worked assiduously to develop the Hashd into an organisation that was neither subject to full prime ministerial command nor subordinate to the conventional security forces," said Knights, as cited by AFP news agency.

Although he worked under Faleh al-Fayyadh, also Iraq's national security adviser, al-Muhandis was widely recognised as the Hashd's "real" leader, observers said.

He had both the utmost loyalty of its forces on the ground and control over its financial resources.

That made him "the central nervous system" of the IRGC's Quds Force in Iraq, Knights wrote last year.

He was a personal adviser to Soleimani, with the two pictured on multiple occasions in warm embraces.

Like the Iranian commander, al-Muhandis sported a white beard and kept his white hair swept into a neat coiffe.

Despite his high-profile position within the Hashd, al-Muhandis rarely appeared in public or delved into politics.

He broke his usual silence last year to blame the US and Israel for a string of mysterious blasts on Hashd bases.

It remains unclear who could replace him, Smyth said, as it would be challenging to find someone with such a close ideological and personal relationship to Iran.