Pro-Assad alliance threatens to hit US positions in Syria

A military alliance fighting in support of President Bashar al-Assad said on Wednesday it could hit US positions in Syria, warning that its "self-restraint" over US air strikes on government forces would end if Washington crossed "red lines".

The threat marks an escalation of tensions between the United States and Iran-backed forces over control of Syria's southeastern frontier with Iraq, where the US has been training Syrian rebels at a base inside Syrian territory.

The United States launched air strikes on Tuesday against what it said were Iranian-backed fighters who it said posed a threat to US and US-backed forces in the area, the second such attack in three weeks.

The statement from the pro-Assad alliance was issued in the name of the "commander of the operations room of the forces allied to Syria", and was circulated by a military news unit run by the Lebanese group Hezbollah, one of Assad's military allies.

Assad's allies also include Iran and Russia. The statement did not spell out whether Moscow was a signatory to it.

"America knows well that the blood of the sons of Syria, the Syrian Arab Army, and its allies is not cheap, and the capacity to strike their positions in Syria, and their surroundings, is available when circumstances will it," the statement said.

It said such attacks could be carried out with "different missile and military systems, in the light of the deployment of American forces in the region".

The statement added that the silence of "the allies of Syria" thus far was not a sign of weakness but "an exercise in self-restraint" to allow for "other solutions".

"This will not last if America goes further, and crosses the red lines," it said.

More than 320,000 people have been killed since civil war erupted in Syria in 2011.

It began with anti-government protests but has since evolved into a complex multi-front war involving the army and rebel groups.