Warplanes from a US-led coalition have struck Syrian pro-regime forces in the country's southeast, officials have said.

The airstrikes, which killed Syrian government forces and Iranian-backed militia, occurred close to the border with Iraq.

US defence secretary Jim Mattis denied that the destruction of the convoy, which included a number of tanks, marked an escalation in the American role in Syria.

"We are not increasing our role in the Syrian civil war," he said. "But we will defend our troops."

In a statement on Twitter, Operation Inherent Resolve said: 

May 18 #Coalition struck #Syrian pro-regime forces advancing in a de-confliction zone near At Tanf posing a threat to #US partner forces1/3

— Inherent Resolve (@CJTFOIR) May 18, 2017

"This was despite Russian attempts to dissuade pro-regime movement towards al Tanf, coalition aircraft show of force and warning shots.

"Coalition forces have operated in the al Tanf area for many months training and advising vetted partner forces who are fighting ISIS."

The claim that Russia was warned and was still unable to stop the convoy will call into question how much influence their commanders have over Assad and Iranian forces in parts of Syria.

US and UK special forces have been training Syrian rebels at a heavily fortified base in al Tanf.

Islamic State attacked the base last month and recently regime forces have been pushing forward nearby, raising fears of a collision with coalition forces.

A 34-mile defensive zone exists around the base, and it is believed these forces strayed inside that.

Iranian troops are said to be commanding the pro-Assad forces and a number of Lebanese Hezbollah fighters.

Exact casualties figures have not been provided, although some reports estimate 50 were killed.