Beirut: A military alliance fighting in support of President Bashar Al 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 United States 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-Al 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 Al Assad’s military allies.

Al 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.

US-led coalition forces in Syria are reinforcing their garrison near the crucial Al Tanf border crossing amid a buildup of pro-government troops nearby.

“We have increased our presence and our footprint and prepared for any threat that is presented by the pro-regime forces,” Colonel Ryan Dillon, spokesman for the coalition in Baghdad, told journalists by video conference earlier this week.

Al Tanf, on the key highway connecting Damascus with Baghdad, has been menaced by a surge of Iran-backed troops loyal to Al Assad. The area, also just northeast of the Jordanian border, is used by coalition forces as a training and staging area for attacks against Daesh.

Coalition planes on May 18 pounded the front of a government convoy that had apparently been headed toward the Al Tanf garrison.

That strike occurred inside an established “deconfliction zone” covering a 55 kilometre radius around the base. The Pentagon said it appeared that the Syrian government troops were trying to build a “fire base” for artillery units inside the zone.

The deconfliction zones are agreed upon between Russia, which is supporting Al Assad, and the coalition, and are designed to stop either side inadvertently striking the other’s forces on the ground and in the air.

Last weekend coalition planes dropped leaflets warning the pro-regime troops to stay away.

Dillon said there are about 200 coalition and partner forces in the area and that support for them has been beefed up.

“We have constant coverage over our forces there in Al Tanf,” he said.

The development comes in the context of growing tension over which forces will take on Daesh in Syria’s east.

Al Assad’s army is trying to prevent US-backed forces from leading that fight.