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By David Gritten
BBC News
The US says it has carried out air strikes on facilities in eastern Syria used by groups affiliated with Iran's Islamic Revolution Guard Corps (IRGC).
Nine ammunition and storage bunkers in Deir al-Zour province were hit early on Wednesday, US military officials said.
Activists said six guards were killed, while Iran denied links to the targets.
President Joe Biden ordered the strikes "to defend and protect US forces" in Syria following several attacks on them blamed on Iran-backed groups.
On 15 August, two drones attacked the al-Tanf Garrison, in the southern desert, and rockets landed near the Green Village base in Deir al-Zour. Neither attack caused any casualties.
About US 1,000 troops operate out of the bases along with allied local fighters as part of the US-led global coalition against the jihadist group Islamic State (IS).
They are tasked with ensuring the total defeat of IS, whose militants once controlled large swathes of Syria and neighbouring Iraq.
The US strikes were conducted by two F-15 and two F-16 jets, military officials told US media.
The plan had been to hit 11 bunkers, but the pilots did not drop bombs on two of them because people were seen walking nearby, they added.
"The United States took proportionate, deliberate action intended to limit the risk of escalation and minimise the risk of casualties," US Central Command spokesman Col Joe Buccino said.
The Syrian Observatory for Human Rights reported that the jets targeted warehouses in Ayyash, a town on the River Euphrates about 10km (6 miles) north of Deir al-Zour city, and a camp belonging to the Fatemiyoun Brigade, an Iran-backed militia made up largely of Shia Muslim Afghan fighters that has supported the Syrian army in the country's civil war.
The UK-based monitoring group posted video footage of a large explosion in Ayyash and cited sources as saying that the strikes killed six Syrian and non-Syrian fighters guarding the sites.
Iranian foreign ministry spokesman Nasser Kanaani insisted the sites targeted "had no links to the Islamic Republic" and called the attack "a violation of Syria's sovereignty and territorial integrity".
The strikes come a day after Iranian state media reported that an IRGC general had been killed while "on a mission" in Syria.
They said Abolfazl Alijani was working as a military adviser but gave no details about the circumstances of his death.
The IRGC has built a substantial presence in Syria since the civil war began in 2011, sending hundreds of troops to advise President Bashar al-Assad's forces and train pro-government militias.
The US supports opposition groups trying to end Mr Assad's rule, but it has sought to avoid direct military intervention in the conflict.