A spokesman said no enemy fire was involved and the cause of the accident is being investigated.

He added the service members are receiving treatment and 10 have been moved to "higher care facilities" outside the region.

There are at least 900 US forces personnel in Syria on average, along with an undisclosed number of contractors.

US special operations forces also move in and out of the country but are usually in small teams and are not included in the official count.

US forces have been in Syria since 2015 to advise and assist the Kurdish-led Syrian Democratic Forces in the fight against the militant Islamic State (IS) group.

IS swept through Iraq and Syria in 2014 and took control of large swathes of territory.

US troops have been trying to prevent any comeback by IS since the extremist group was defeated in Syria in March 2019.

IS sleeper cells remain a threat and there are also about 10,000 IS fighters held in detention facilities in Syria.

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Tens of thousands of their family members are living in two refugee camps in the country's northeast.

Over recent years, US troops have been subject to attacks carried out by IS members and Iran-backed fighters.

In late March, a drone attack on a US base killed a contractor and wounded five American troops and another contractor.

In retaliation, US fighter jets struck several locations around the eastern province of Deir el Zour, which borders Iraq.

US defence secretary Lloyd Austin said at the time that the strikes were a response to the drone attack as well as a series of recent attacks against US-led coalition forces in Syria by groups affiliated with Iran's Revolutionary Guard.

In a related development, Syrian Kurdish-led authorities announced on Saturday that hundreds of IS fighters held in prisons around the region will be put on trial after their home countries refused to repatriate them.