Iran's most powerful military commander, General Qasem Soleimani, has been killed by a US airstrike in Iraq.

The 62-year old spearheaded Iranian military operations in the Middle East as head of Iran's elite Quds Force.

He was killed at Baghdad airport, alongside local Iran-backed militias, early on Friday in a strike ordered by US President Donald Trump.

Gen Soleimani's killing marks a major escalation in tensions between Washington and Tehran.

Under his leadership, Iran had bolstered Hezbollah in Lebanon and other pro-Iranian militant groups expanded Iran's military presence in Iraq and Syria and orchestrated Syria's offensive against rebel groups in the country's long civil war.

Iran's Supreme Leader Ayatollah Ali Khamenei said "severe revenge awaits the criminals" behind the attack. He also announced three days of national mourning.

Gen Soleimani was widely seen as the second most powerful figure in Iran behind the Ayatollah Khamenei. His Quds Force, an elite unit of the Iranian Revolutionary Guards, reported directly to the Ayatollah and he was hailed as a heroic national figure.

But the US has called the commander and the Quds Force terrorists and holds them responsible for the deaths of hundreds of American and coalition troops.

President Trump, who was in Florida at the time of the strike, tweeted an image of the American flag shortly after the news broke.

A statement from the Pentagon said Gen Soleimani "was actively developing plans to attack American diplomats and service members in Iraq and throughout the region".

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

Meanwhile, global oil prices have soared by more than 4% in the wake of the attack.

Gen Soleimani and officials from Iran-backed militias were leaving Baghdad airport in two cars when they were hit by a US drone strike near a cargo area, US media reports say.

The commander had reportedly flown in from Lebanon or Syria. Several missiles reportedly struck the convoy and at least five people are thought to have died.

Iran's Revolutionary Guards said Iraqi militia leader Abu Mahdi al-Muhandis was among those killed.

The Pentagon statement said: "At the direction of the president, the US military has taken decisive defensive action to protect US personnel abroad by killing Qasem Soleimani."

It added: "The United States will continue to take all necessary action to protect our people and our interests wherever they are around the world."

The drone strike comes days after protesters attacked the US embassy in Baghdad, clashing with US forces at the scene. The Pentagon said Gen Soleimani approved the attacks on the embassy.