The United States has warned rebels in southern Syria that they should not expect military intervention if government troops launch an assault against them, a rebel commander told AFP yesterday.

Russian-backed government forces are preparing an offensive to retake Syria's southern provinces of Daraa and Sweida, still mostly held by rebels who had been backed by the US for years.

The US warning, contained in an Arabic-language message seen by AFP, came as Syrian troops this week stepped up air strikes on the southern provinces.

"We must clarify our position: we understand that you must make a decision (to fight) based on your interests, the interests of your people and your faction as you see them," the message read.

"You should not base your decision on an assumption or expectation of military intervention from our side," it said, according to a copy provided to AFP by one of the rebels."

The US did not immediately confirm the letter's contents.

On Saturday, Syrian regime forces made their first gains on the ground against rebel fighters in Daraa province after several days of intensified air strikes, according to a war monitor.

The Britain-based Syrian Observatory for Human Rights also reported that Russia late Saturday carried out its first air strike on Daraa since brokering a truce there in mid-2017.