A monitoring group says Russian jets have struck rebel-held areas in southern Syria in the first such attacks since Moscow agreed to a cease-fire in July 2017, endangering one of the so-called "de-escalation" zones.
"Intense Russian air strikes are hitting towns in Daraa's eastern countryside for the first time since the cease-fire was agreed in southern Syria last year," said Rami Abdel Rahman, head of the British-based Syrian Observatory for Human Rights.
Reuters reported that two tracking centers which monitor military aircraft movements recorded at least 20 strikes on Busra al-Harir, northeast of the city of Deraa.
Opposition sources said Russian jets struck a rebel-held town in southwest Syria early on June 24, providing air cover to an expanding campaign by Syrian Army units to recapture the strategic area on the border with Jordan and the Israeli-occupied Golan Heights.
Moscow did not immediately comment on the reports.
Russian warplanes had not previously been deployed in the current offensive, with Syrian government troops relying on artillery and rockets in the assault.
Russia, which along with Iran backs Syrian President Bashar al-Assad in the seven-year civil war, has in other areas played a key role in the success of government forces' campaigns by providing key air cover.
Assad has vowed to recapture the region, and his army last week ramped up its efforts.
Reuters reported that a message sent by Washington to the Free Syrian Army (FSA) said the rebels should not expect help from the U.S. military.
The message said the U.S. government wanted to make clear that "you should not base your decisions on the assumption or expectation of a military intervention by us."
"We in the United States government understand the difficult conditions you are facing and still advise the Russians and the Syrian regime not to undertake a military measure that violates the [de-escalation] zone," the message also said.
Before reports of the Russian air strikes, the United States on June 22 warned Russia to rein in its ally Syria after reports that government forces had stepped up attacks on rebels in an area designated as a "de-escalation zone."
U.S. Ambassador to the United Nations Nikki Haley said the Syrian military escalation "unambiguously violates" the de-escalation arrangement and that the bombardments have already prompted more than 11,000 civilians to flee the area.
"We expect Russia to do its part to respect and enforce the cease-fire it helped establish, and to use the influence it has to stop the Syrian regime's violations and any further destabilizing actions in the southwest and throughout Syria," she said.
The United States backs antigovernment rebels fighting Assad’s government in the war, which began in 2011 with a government crackdown against pro-democracy protesters.
Islamic State (IS) militants and other groups also joined the war, opposed by all other sides, but have mainly been driven from all of their strongholds in the country.