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Increased rocket and drone strikes on US bases in eastern Syria, linked to the war in Gaza, underscore Iran's reach in one of the most strategic theatres in its rivalry with Washington, officials have said.
A retaliatory US air strike on Sunday was said to have killed several Iran-backed fighters, according to the Syrian Observatory for Human Rights, a war monitor.
A US defence official told The National they were not yet aware of fatalities as a battle damage assessment is ongoing.
As the US deployed naval forces in the Mediterranean – including two carrier groups capable of launching hundreds of air strikes a day – Tehran has not acted on its rhetoric of opening a new front against Israel.
But Iran's President Ebrahim Raisi has publicly said armed groups in the region must back Hamas. Pro-Iranian militias have been attacking US forces in Syrian areas east of the Euphrates River and to the south, near the border with Jordan and Iraq, according to the US military.
“No American soldiers have been killed but they are facing a major escalation,” said an ex-Syrian army officer, who has been collecting reconnaissance from the east on behalf of the opposition to President Bashar Al Assad.
He said the militias “are appearing heroic” by partly fulfilling their promise to support Hamas, the Iran-backed group that started the Gaza war by attacking southern Israel on October 7.
Four American bases have been the target of drones, mostly flown from Iraq, as well as mortar bombs and multiple rocket launcher systems fired from Syria, he said.
The US military has recorded 40 attacks in Syria and on bases in Iraq that have wounded 45 personnel, with “traumatic brain injuries” among the injuries.
Syria’s east is the centre of the country’s oil production, which mostly falls in areas controlled by American forces, which number about 700.
The region also constitutes the main corridor for the transport of weapons from Iran to the Lebanese guerrilla group Hezbollah, Tehran's most powerful militia ally, according to American, Israeli and Arab officials.
Lloyd Austin warning
US Defence Secretary Lloyd Austin said on Monday that Washington will “do what's necessary” to protect American troops.
He was speaking a day after US warplanes raided positions in Syria that Washington said were being used by Iran’s Islamic Revolutionary Guard Corps. It was the third such raid in less than three weeks.
Like most of Syria, different players have exchanged control of the east since the civil war, which broke out in 2011.
The war started with peaceful demonstrations against the 23-year rule of Mr Al Assad. Authorities suppressed the pro-democracy protest movement and the revolt became militarised.
The 2015 Russian intervention was crucial for the President to remain in power, but large areas of the country fragmented into Iranian, Russian, Turkish and US zones of control.
Many areas west of the Euphrates, especially the Albu Kamal border crossing with Iraq, are firmly with pro-Iranian militias, despite the mostly nominal presence for the Syrian military, according to the ex-officer and a Kurdish source working with US forces across the river.
About half of the 7,000 Iranian and pro-Iranian fighters in the east are concentrated along a stretch of the Euphrates from Albu Kamal to the town of Al Qitaa, they said.
The fighters are overseen by the IRGC, which also has members of its elite Al Quds Force in the area, the sources said. Hezbollah primarily oversees smuggling routes through Albu Kamal and supervises local tribesmen who man positions along the border with Iraq, the sources said.
The main Iraqi militias in the area belong to the Popular Mobilisation Units, the main Shiite militia group that has great influence over the government in Baghdad and openly admits its presence in Syria.
The PMU units maintain security in the towns and villages that form the Iranian-controlled belt and recruit auxiliaries from the area's clans and tribes, according to sources close to the PMU in Baghdad.
They are assisted by Liwa Al Fatemiyoun, mostly comprising Shiites from Afghanistan, and the Pakistani Liwa Al Zainabiyoun, illustrating the degree to which Iran has integrated international fighters into a single command in charge of a crucial passageway in Syria.
For now, it is unclear the extent the violence could escalate. Elements of these militias, supported by Russian mercenaries from the Wagner Group, launched a major ground attack on US and Kurdish positions in 2018, near the formerly Conoco-run oilfield, which is still called Conoco.
The US responded with massive air strikes, killing about 100 of the attackers, while one Kurdish militiaman was injured.
Since then, militia attacks have been limited, and in comparison to the battle of Khasham, US retaliation has been restrained.
But analysts warn this could spiral out of control, as the frequency of militia attacks signals a determination to kill or drive out US forces.
Additional reporting by Sinan Mahmoud in Baghdad