BAGHDAD - The United States killed Iran's top general and the architect of Tehran's proxy wars in the Middle East in an airstrike at Baghdad's international airport on Friday, an attack that threatens to dramatically increase tensions in the region.

The targeted killing of General Qassem Soleimani, the head of Iran's elite Quds Force, could draw forceful Iranian retaliation against American interests in the region and spiral into a far larger conflict between the US and Iran, endangering US troops in Iraq, Syria and beyond.

The US Defence Department said it killed Soleimani because he "was actively developing plans to attack American diplomats and service members in Iraq and throughout the region". It also accused Soleimani of approving the attacks on the US Embassy in Baghdad earlier this week.

A vehicle burns at Baghdad airport after the US airstrike that killed General Qassem Soleimani, the head of Iran's elite Quds Force, at the direction of President Donald Trump (Iraqi Prime Minister's Press Office/PA)
An adviser to Iran's President Hassan Rouhani quickly warned US President Donald Trump of retaliation from Tehran.

"Trump through his gamble has dragged the US into the most dangerous situation in the region," Hessameddin Ashena wrote on the social media app Telegram. "Whoever put his foot beyond the red line should be ready to face its consequences."

The airport strike also killed Abu Mahdi al-Muhandis, deputy commander of Iran-backed militias in Iraq known as the Popular Mobilisation Forces, and five others including the PMF's airport protocol officer, Mohammed Reda, Iraqi officials said.

Mr Trump was holidaying on his estate in Palm Beach, Florida, but sent out a tweet of an American flag.

The dramatic attack comes at the start of a year in which Mr Trump faces both a Senate trial following his impeachment by the US House of Representatives, and a re-election campaign. It marks a potential turning point in the Middle East and represents a drastic change for American policy toward Iran after months of tensions.

Tehran shot down a US military surveillance drone and seized oil tankers. The US also blames Iran for a series of attacks targeting tankers, as well as a September assault on Saudi Arabia's oil industry that temporarily halved its production.

The tensions take root in Mr Trump's decision in May 2018 to withdraw the US from Iran's nuclear deal with world powers, struck under his predecessor, Barack Obama.

Soleimani was the target of Friday's US attack, which was conducted by an armed American drone, according to a US official. His vehicle was struck on an access road near the Baghdad airport.

A senior Iraqi security official said the airstrike took place near the cargo area after Soleimani left his plane to be greeted by al-Muhandis and others. The official said the plane had arrived from either Lebanon or Syria.

Two officials from the PMF, who spoke on condition of anonymity, said Suleimani's body was torn to pieces in the attack, while they did not find the body of al-Muhandis. A senior politician said Soleimani's body was identified by the ring he wore.

It's unclear what legal authority the US relied on to carry out the attack. American presidents claim broad authority to act without congressional approval when US personnel or interests are facing an imminent threat. The Pentagon did not provide evidence to back up its assertion that Soleimani was planning new attacks against Americans.

Democratic Connecticut Sen. Richard Blumenthal said Mr Trump owed a full explanation to Congress and the American people.

"The present authorisations for use of military force in no way cover starting a possible new war. This step could bring the most consequential military confrontation in decades," Blumenthal said.

But Trump allies were quick to praise the action.

"To the Iranian government: if you want more, you will get more," tweeted South Carolina Sen. Lindsey Graham.

For Iran, the killing represents more than just the loss of a battlefield commander, but a cultural icon who represented national pride and resilience while facing US sanctions.

While careful to avoid involving himself in politics, Soleimani's profile rose sharply as US and Israeli officials blamed him for Iranian proxy attacks abroad.

While Iran's conventional military has suffered under 40 years of American sanctions, the Guard has built up a ballistic missile program. It also can strike asymmetrically in the region through forces like Lebanon's Hezbollah and Yemen's Houthi rebels. The US has long blamed Iran for car bombings and kidnappings it never claimed.

As the head of the Quds, or Jersualem Force of Iran's paramilitary Revolutionary Guard, Soleimani led all of its expeditionary forces and frequently shuttled between Iraq, Lebanon and Syria.

US officials say the Guard under Soleimani taught Iraqi militants how to manufacture and use especially deadly roadside bombs against US troops after the invasion of Iraq. Iran has denied that. Soleimani himself remained popular among many Iranians, who saw him as a selfless hero fighting Iran's enemies abroad.