Russian jets struck an opposition-held town in southwest Syria on Sunday, opposition sources said, in the first air cover provided by Moscow to an expanding Syrian army offensive to recapture the strategic area bordering Jordan and the Israeli occupied Golan Heights.
Two tracking centres that monitor military aircraft movements recorded at least twenty strikes on Busra al Harir, northeast of Deraa, 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 government 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.
Syrian President Bashar al-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.
Throwing in Russia's full military weight in the campaign to regain southern Syria will weaken the ability of mainstream Free Syrian Army (FSA) rebel groups to withstand relentless bombing on civilian areas that forced their compatriots in other areas to submit to surrender deals.
Opposition hopes had been raised after Washington warned Assad and his Russian allies that violations of the zone would have "serious repercussions" and pledged "firm and appropriate measures".
U.S. Ambassador to the United Nations Nikki Haley said earlier on Friday that the Syrian military escalation "unambiguously violates" the de-escalation arrangement.
"Russia will ultimately bear responsibility for any further escalations in Syria," Haley said in a statement.
Rebels in southern Syria have received support, including arms from Assad's foreign foes during the seven-year-long war.
But analysts believe the aid has dropped after U.S. President Donald Trump decided last year to shut down a military aid program run by the Central Intelligence Agency.
Unless an agreement is reached between Moscow and Washington over the fate of southern Syria, a big offensive risks an escalation that could draw the United States deeper into the war.
Southwest Syria is of strategic concern to U.S.-allied Israel, which this year has stepped up attacks on Iran-backed militia allied to Assad.
U.S 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.