While the US has warned the Syrian regime over its actions on a deescalation zone in the south of the war-torn country, any US military intervention has been ruled out by the Trump administration.
The United States has told Syrian rebel factions they should not expect military support to help them resist a Russian-backed regime offensive to regain opposition-held parts of Syria bordering Jordan and the Israeli-occupied Golan Heights.
A copy of a message sent by Washington to heads of Free Syrian Army (FSA) groups, which was seen by Reuters, said the US government wanted to make clear that "you should not base your decisions on the assumption or expectation of a military intervention by us".
The United States had earlier warned Syrian regime leader Bashar al Assad and his Russian allies that violations of a "de-escalation" zone agreed by the United States and Russia last year would have "serious repercussions" and pledged "firm and appropriate measures."
The toughly worded statements coming from the administration had raised the hopes of the Western-backed opposition of a possible American military intervention in the event the Syrian army's several days long bombing campaign broadens to an all-out offensive across the southwest.
Left alone to decide
The US message also told the rebels it was left to them alone to take the right decision on how to face the Syrian regime's military campaign based on what they saw was best for themselves and their people.
"We in the United States government understand the difficult conditions you are facing and still advise the Russians and the Syrian regime not to undertake a military measure that violates the zone," the message also said.
The US has supported the moderate mainstream FSA faction with millions of dollars worth of arms and paid monthly salaries to thousands of rebels in the course of the seven-year war under a military aid programme run by the Central Intelligence Agency.
But analysts believe the aid has dropped after US President Donald Trump decided last year to shut down the programme.
Russian jets strike
Late on Saturday, Russian jets struck an opposition held town in southwest Syria, opposition sources said, in the first air cover provided by Moscow to an expanding Syrian regime offensive.
Two tracking centres that monitor military aircraft movements recorded at least twenty strikes on Busra al Harir, northeast of Daraa, a city in Syria, two sources told Reuters.
"We have tracked a sortie of 5 Russian jets that performed 25 raids," said one source saying the war jets had set off from Russia's Hmeimim air base in the western coastal province of Latakia in Syria.
Syrian regime forces had so far made heavy use of artillery and rockets in the current assault, and Russian warplanes that were critical to the recovery of other rebel-held areas had not been deployed until now.
Assad has sworn to recapture the sensitive strategic area and the army began ramping up its assault there last week, threatening a "de-escalation" zone agreed by the United States and Russia last year.
Since the start of the offensive last week, the Syrian regime had mostly deployed artillery and rockets. Russian warplanes that were critical to the recovery of other rebel-held areas were conspicuously absent.
Throwing in Russia's full military weight in the campaign to regain southern Syria will weaken the ability of mainstream rebel groups to withstand relentless bombing on civilian areas that forced their compatriots in other areas to submit to surrender deals.
Jordan wants zone preserved
US ally Jordan which had long said the zone had brought relative calm on its northern border announced it was engaged in intensive diplomacy with Washington and Moscow to preserve the zone and prevent a wider confrontation.
"We stress the importance of respecting the agreement and we are working to prevent the explosion of the situation," Jordan's Foreign Minister Ayman Safadi said in a tweet on Friday.
The southwest is of strategic concern to US-allied Israel, which has this year stepped up attacks on Iran-backed militia allied to Assad.
US ally Jordan, which has been worried by the escalation, said it was engaged in intensive diplomacy with Washington and Moscow to preserve the zone and prevent a wider confrontation.
The loss of opposition-held southern Syria would deal a major blow to the rebel cause.
The southwestern city of Daraa is seen by the opposition as the cradle of the 2011 uprising that began as a peaceful protest movement against Assad's authoritarian rule but has spread across the country and degenerated into civil war.