Warplanes struck an opposition-held area in Syria's southwestern Deraa province on Tuesday, a war monitor and local activists reported, a military escalation in territory where government forces are threatening a major offensive.
The raids hit the area of al-Masika village in northeastern Deraa province. Clashes were also underway in the area.
Rebel forces control swathes of territory in southwestern Syria, which borders Jordan and the Israeli-occupied Golan Heights.
Syria's multi-sided war has pivoted towards the southwest in recent weeks, risking escalation in an area of major concern to Israel where the conflict has been contained since last year by an agreement underwritten by the United States and Russia.
While a notable escalation, Rami Abdulrahman, director of the Syrian Observatory for Human Rights, said the air strikes did not yet appear to mark the start of the big offensive that government forces and their allies have been mobilising for.
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The United States last week warned it would "take firm and appropriate measures" in response to government violations of the so-called "de-escalation" agreement in the southwest.
Israel has been demanding that Iranian and Iran-backed forces such as Lebanon's Hezbollah be kept away from the Golan frontier, and removed from Syria more widely.
President Bashar al-Assad said earlier this month he was giving a chance for political talks to resolve the situation in the southwest, though if these failed the government would resort to force to take back the territory.