US-Iran tensions have peaked with the assassination of Qassem Soleimani, a 62-year-old who headed the foreign arm of the Islamic Revolution Guard Corps (IRGC), Iran's elite military force.

Soleimani was deeply popular at home and among Tehran's allies.

He survived several previous assassination attempts over the past 20 years and was credited with helping armed groups defeat the Islamic State of Iraq and the Levant (ISIL or ISIS) group.

More:

Here are five things to know:

What has happened?

At the direction of US President Donald Trump, the US military killed Soleimani, head of Iran's Quds Force, the foreign arm of the IRGC, in the early hours of Friday, January 3, 2020, in an air raid near the cargo area of Baghdad International Airport.

Soleimani had been travelling in a car when the missiles hit, reportedly having flown into Iraq from Lebanon or Syria.

200103033905377

The Islamic Revolution Guard Corps (IRGC) said 10 people were killed, including five of its members including Soleimani. Abu Mahdi al-Muhandis, deputy leader of Hashd al-Shaabi, or Popular Mobilisation Forces (PMF) - an Iran-backed Iraqi paramilitary unit, was among the dead, as well as other Iraqi militia members.

The PMF is supported by the IRGC, which trains several groups in the region.

The PMF's Al-Muhandis was seen as Tehran's man in Iraq.

Quds Force is the overseas arm of the IRGC, which has an estimated 150,000 active personnel.

It was set up after the Iranian Revolution in 1979 by order of then-supreme leader, Ayatollah Ruhollah Khomeini, conceived as the defending force of the uprising.

It is separate from the Iranian Army, has close ties to the current supreme leader Ayatollah Ali Khamenei, and is designated by the US as a "foreign terrorist organisation".

Why did the US assassinate Soleimani?

US-Iran tensions have been rising since Washington pulled out of a landmark nuclear agreement with Tehran last year and began reimposing punishing sanctions.

In recent weeks, those tensions have escalated.

On December 27, an American contractor was killed in a rocket attack in Iraq - which the US blamed on Iran-backed Kataib Hezbollah, a militia belonging to PMF. The US responded on December 29 by targeting sites belonging to Kataib Hezbollah in Iraq and Syria, killing at least 25 fighters.

200103075922283

On December 31, a rare protest unfolded at the US embassy compound in Baghdad, a heavily fortified area, with demonstrators who sympathised with or belonged to PMF attempting to vandalise the embassy.

In a statement after the assassination, the US Department of Defence said Soleimani was developing plans to attack American diplomats and military members throughout the Middle East region.

It blamed him and the Quds Force for the deaths of "hundreds of American and coalition service members", as well as the December 27 death of the US contractor and the heated embassy protest.

What has Iran said?

Calls for revenge are growing louder in Iran.

Soleimani was said to have been so close to the supreme leader that he reported directly to him.

Ayatollah Khamenei said in a statement carried by state media: "All enemies should know that the jihad (struggle) of resistance will continue with a doubled motivation, and a definite victory awaits the fighters in the holy war."

Iranian President Hassan Rouhani tweeted: "The great nation of Iran will take revenge for this heinous crime."

200103100607193

Mohsen Rezaei, former IRGC commander, said on Twitter: "[Soleimani] joined his martyred brothers, but we will take vigorous revenge on America."

Mohammad Javad Zarif, Iran's foreign minister, tweeted his response, saying: "The US' act of international terrorism, targeting & assassinating General Soleimani - THE most effective force fighting Daesh (ISIS), Al Nusrah, Al Qaeda et al - is extremely dangerous & a foolish escalation."

How has the rest of the world reacted?

World leaders and international groups have called for restraint. You can find all the main reactions here.

While Iraq, Syria and Russia condemned the US for the killing, Turkey said Ankara "has always been against foreign interventions, assassinations and sectarian conflicts in the region".

Israeli Prime Minister Benjamin Netanyahu, meanwhile, said the US had the right to defend itself by killing Soleimani.

The United Kingdom called for de-escalation but said it has "always recognised aggressive threat posed by the Iranian Quds Force led by Qassem Soleimani."

What's expected to happen now?

Soleimani's body is expected to arrive in Iran on Saturday and he will finally buried in his native Kerman Province in the country's southeast on Tuesday, following a significant three-day funeral ceremony that is expected to be on a similar scale to Ayatollah Ruhollah Khomeini's in 1989. On Sunday, rituals will take place in the holy city of Mashhad. His body will then be taken to Tehran on Monday.

The Iraqis who died in the attack will be buried on Saturday in Baghdad, according to the PMF, in funerals that thousands are expected to attend.

The assassination has raised fears of a military conflict between Iran and the US, that might play out in countries such as Iraq and Syria.

200103122412024

The US Department of Defense said in its statement that the killing was "aimed at deterring future Iranian attack plans" as it warned it would take "all necessary action" to protect Americans and the country's interests around the world.

A report citing anonymous US officials said the US was expected to send 3,000 extra troops to the region.

Tehran has made it clear it is seeking revenge.

"Iraq is bracing for some of the most difficult days," Baghdad-based analyst Jassim Moussavi told Al Jazeera. "We expect the announcement of war at any moment. If Iran decides to confront the US, Iraq will be the scene for that battle. Several Shia paramilitary forces have started to prepare themselves for ground zero."

Ali Akbar Dareini, an expert on Iran-US affairs at the Center for Strategic Studies in Tehran, believes the attack will lead to more insecurity and violence across the Middle East.

"This is also a gift to [ISIL] and all terrorists in the region," the political analyst told Al Jazeera.