AMMAN, JORDAN - 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 centers that monitor military aircraft movements recorded at least 20 strikes on Busra al-Harir, northeast of Deraa city, two sources told Reuters.

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 to by the United States and Russia last year.

The United States on Thursday reiterated its demand that the zone be respected, warning Assad and his Russian allies that violations would have "serious repercussions" and accusing Damascus of initiating airstrikes, artillery and rocket attacks.

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.