The United States warned Syria on Friday it would take "firm and appropriate measures" in response to ceasefire violations, saying it was concerned about reports of an impending military operation in a de-escalation zone in the country's southwest.
Washington also cautioned Syrian President Bashar al-Assad against broadening the conflict.
"As a guarantor of this de-escalation area with Russia and Jordan, the United States will take firm and appropriate measures in response to Assad regime violations," State Department spokeswoman Heather Nauert said in a statement late on Friday.
A war monitor, the Britain-based Syrian Observatory for Human Rights, reported on Wednesday that Syrian government forces fresh from their victory this week against an Daesh pocket in south Damascus were moving into the southern province of Deraa.
Syrian state-run media have reported that government aircraft have dropped leaflets on opposition-held areas in Deraa urging fighters to disarm.
Backed by Russian warplanes, ground forces from Iran and allied militia, including Lebanon's Hezbollah, have helped Assad drive the opposition from Syria's biggest cities, putting him in an unassailable military position.
They have recaptured all remaining opposition areas near Damascus in recent weeks, including the densely populated eastern Ghouta area, as well as big enclaves in central Syria.
The government is now in its strongest position since the early months of the war in 2011, although still a long way from achieving Assad's aim of reasserting sway over all of Syria.
Anti-Assad opposition still control two large contiguous areas of territory.
The government's gains have brought it to a point where any new military campaign risks putting it in conflict with foreign powers.
