Joe Biden vowed that Washington "shall respond" to the attack on US forces in northeast Jordan near the Syrian border on Saturday, which also left dozens of troops injured.

The president blamed Iranian-backed militias for the first US fatalities after months of strikes by such groups against American forces across the Middle East.

Iran said "resistance groups" do not take orders from the Islamic Republic.

Nevertheless, it is known that a number of groups across the region have received Iranian support.

Some of these - like Hamas, Hezbollah and the Houthis - have been discussed in detail in other Sky News stories, but others are less well known.

Islamic Resistance in Iraq

Responsibility for the deadly attack on US forces over the weekend was claimed by Islamic Resistance in Iraq, an umbrella term used by a number of Iran-backed groups in Iraq.

Groups under their banner have been involved in armed operations against US troops based in Iraq and Syria, stemming from America's role in the Israel-Hamas war.

The use of the generic brand may help to obscure exactly which group is carrying out a given attack, according to the Washington Institute, a thinktank based in Washington DC.

The thinktank argues that the fact they are coordinating under one brand is significant, as ordinarily they tend to "jealously guard" their individual identities and credit for attacks.

"Their willingness to submerge these identities... suggests that a higher power is coordinating them," it added, pointing the finger at Iran's Islamic Revolutionary Guards Corps (IRGC).

The Islamic Resistance in Iraq is believed to include groups such as:

- Tashkil al-Waritheen (The Inheritors)
- Kataib Hezbollah (Battalions of the Party of God)
- Asaib Ahl al Haq (League of the Righteous)
- Harakat Hezbollah al Nujaba (Movement of the Party of God's Noble Ones)
- Kataib Sayyid al Shuhada (Masters of the Martyrs Brigade)
- Badr Organisation

Zaynabiyoun Brigade (Syria)

The Zaynabiyoun Brigade is a Shia militia established and trained by the Revolutionary Guards and its branches.

Its origins can be traced back to attacks against the Sayyidah Zaynab mosque in Damascus in 2015, which was targeted by Sunni militants, for which some of the first Pakistani Shias were recruited by Iran to fight, according to the India-based Observer Research Foundation.

It arose around the time of the Syrian civil war and was involved in fighting against Islamic State.

In 2019, the group was sanctioned by the US Treasury for supporting the IRGC's Qods Force and human rights abuses in Iran.

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Fatemiyoun Division (Syria)

Dating back to the 1980s, the Fatemiyoun Division is an Afghan militia mostly made up of Hazara refugees from Afghanistan living in Iran.

Since 2014, it has been deployed by Iran's IRGC to fight in Syria alongside the Assad regime's forces

Similar to the Zaynabiyoun Brigade, the Fatemiyoun Division was sanctioned by the US Treasury in 2019 for supporting the IRGC's Qods Force and human rights abuses in Iran.

It has been accused of involvement in war crimes, with Human Rights Watch alleging that Afghan children as young as 14 have fought in its ranks after being recruited by the IRGC.

Saraya al Ashtar and Saraya al Mukhtar (Bahrain)

Also known as the Al Ashtar brigades, Saraya al Ashtar is based in Bahrain but is considered to be funded, trained and armed by Iran.

It has been accused by the US of carrying out terrorist attacks in an effort to overthrow the Bahraini government.

Similar to other Iran-affiliated militia groups in the region, Saraya al Ashtar has adopted IRGC branding.

Saraya al Mukhtar is another group based in Bahrain and allegedly supported by Iran.

It has been reported that the group's ultimate goal is to overthrow the House of Khalifa - the ruling family in Bahrain - and turn the country into a province of Iran.