The United States warned Syria that it would take "firm and appropriate measures" if President Bashar Al Assad violates a ceasefire deal.
The words of caution came after Syrian aircraft dropped leaflets on the southern province of Deraa ahead of an expected offensive. The province is expected to be among the next targets in Mr Al Assad regime's string of reconquests.
Residents of Deraa said on Friday that several different leaflets were scattered across the province.
One of them, seen by a journalist contributing to AFP in the city of Deraa, includes a picture showing lined up bodies, presumably of anti-government fighters.
"This is the inevitable fate of anyone who insists on carrying arms," reads the leaflet.
Late on Friday the US State Department issued a statement saying it was "concerned" by the reports and that the area in question was within the boundaries of a de-escalation zone it had negotiated with Russia and Jordan last year.
"We also caution the Syrian regime against any actions that risk broadening the conflict or jeopardize the ceasefire," said spokeswoman Heather Nauert, adding that the ceasefire had been re-affirmed by President Donald Trump and his Russian counterpart Vladimir Putin at a meeting in Vietnam in November.
"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," she added.
Syrian regime and allied forces on Monday retook the Yarmouk area in southern Damascus, giving Mr Al Assad full control of the capital and its surroundings for the first time since 2012.
Deraa's location makes any broad operation there very sensitive, with Israel suspecting Damascus' Iranian allies of seeking to establish a military footprint closer to its borders.
Government and allied forces control about 30 per cent of Deraa, the rest of which is held by various factions, including a small contingent of ISIS fighters.
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Meanwhile on Saturday Lebanon expressed concern to Syria over a new law aimed at redeveloping areas devastated by seven years of war, saying the initiative could hinder the return of many Syrian refugees to their homeland.
Lebanese Foreign Minister Gebran Bassil wrote in a letter to his Syrian counterpart Walid Al Moualem that the terms of "Law 10" could make it difficult for refugees to prove property ownership, and in turn discourage some from returning.
The legislation came into effect last month as the army was on the brink of crushing the last insurgent enclaves near Damascus.
It allows people to prove they own property in the areas chosen for redevelopment, and to claim compensation. But aid groups say the chaos of war means few will be able to do so in the time specified. The law has yet to be applied.
Minister Bassil, whose country hosts more than a million Syrian refugees, voiced concern over the limited time frame given for refugees to prove possession of their properties.
"The inability of the refugees to practically present what proves their possession (of their properties) during the given time limit might lead to them losing their properties and their sense of national identity," Mr Bassil said in the letter, according to a Foreign Ministry statement.
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"This would deprive them of one of the main incentives for their return to Syria," he added, echoing comments earlier this week by Lebanese Prime Minister Saad Al Hariri.
Prime Minister Hariri said the law "tells thousands of Syrian families to stay in Lebanon" by threatening them with property confiscation.
Minister Bassil sent a similar letter to UN Secretary-General Antonio Guterres, calling for action to protect the rights of Syrian refugees in maintaining their properties.
