Who are the main actors in eastern Syria?

US forces killed 11 pro-Iran fighters in eastern Syria, a monitor said Friday, after a drone attack claimed the life of an American -- the latest flare-up in an area dominated by foreign-backed actors.

The US military operation occurred in the eastern province of Deir Ezzor, which is bisected by the Euphrates river with a patchwork of rival forces having taken up positions on opposing banks.

AFP unpacks the deployments in a porous region on the Syrian border with Iraq where the United States and its Iran-backed rivals operate in close proximity.

- How big is Iran's footprint? -

Iran-backed paramilitary groups have bolstered President Bashar al-Assad's forces since the start of Syria's civil war in 2011.

Around 15,000 pro-Iran proxy fighters from Iraq, Afghanistan and Pakistan are deployed in Deir Ezzor, according to the Syrian Observatory for Human Rights war monitor.

The border area is a key segment of the route that pro-Iran armed groups use to move fighters, weapons and even consumer goods between Iraq and Syria.

Tehran denies deploying regular troops to Syria but says its Islamic Revolutionary Guard Corps has "military advisers" with pro-regime forces.

Kataeb Hezbollah, a pro-Iran group operating under the umbrella of Iraq's Hashed al-Shaabi network of militias, is one of the best-known.

Iraqi units in Syria also include the Imam Ali Brigades and Sayyed al-Shuhada Brigades.

Together with Kataeb Hezbollah, these forces are known to have especially close ties to the Iran Guards.

Iran's ally, the Lebanese Shiite movement Hezbollah, has also deployed advisers and commanders in the region since it officially announced its military backing for Assad in 2013.

Shiite Afghan militiamen of the Fatimid Brigade and Pakistani fighters with the Zainabiyoun Brigade are also stationed in Deir Ezzor, according to the Observatory.

The Fatimid Brigade is considered to be one of the largest pro-Iran groups in Syria, according to the Observatory, and along with the Zainabiyoun Brigade has been hit by US sanctions since 2019.

Thousands of Syrians are also in Iran's pay and have joined local militias in Deir Ezzor.

- Where are US forces based? -

The US backs the Kurdish-led Syrian Democratic Forces, the de facto army of semi-autonomous Kurdish administration, which oversees areas east of the Euphrates.

The SDF seized territory there following successive US-backed campaigns against the Islamic State group which it declared defeated in the east Syria hamlet of Baghouz in 2019.

US-led coalition forces, which entered Syria in 2014 to fight IS have set up bases in the Al-Omar oil field, the country's largest, as well as the Conoco gas field -- both of which lie on the eastern banks of the Euphrates.

US personnel are also stationed in the Kurdish-controlled Hasakeh region in Syria's northeast and the northern province of Raqa, also under Kurdish control.

In 2016, they set up a remote base in the strategic Al-Tanf region in southern Syria, near the borders with Jordan and Iraq.

- Occasional flare-ups? -

The volatile region that is home to some of Syria's most significant oil fields and gas plants has been at the centre of flare-ups between Iran and its American and Israeli rivals.

Israeli raids regularly target weapons shipments, arm depots and military outposts operated by Iran-backed groups in Syria's east.

In November 2022, an Israeli strike killed at least 14 people when it hit a pro-Iran militia truck convoy carrying arms and fuel, the Observatory said.

The next month, Israel's then-military chief Aviv Kohavi said his country had launched the raid, adding the convoy had been carrying weapons bound for Hezbollah in Lebanon.

Washington too has claimed raids there.

On Friday the Observatory said 11 pro-Iran fighters were killed in US air strikes on Syria carried out in retaliation for a drone attack that left an American dead and wounded six others.

A US contractor was killed, and another contractor and five US service personnel were wounded, when a kamikaze drone "of Iranian origin" struck a maintenance facility on a base of the US-led coalition near Hasakeh, the Pentagon said of the incident.

US Defense Secretary Lloyd Austin said Thursday that, at President Joe Biden's direction, he had ordered in response "precision air strikes tonight in eastern Syria against facilities used by groups affiliated with Iran's Islamic Revolutionary Guard Corps".