Russian warplanes have bombarded rebel-held areas of in the country's southwest in the first air support by Moscow in a widening regime offensive to retake the strategic zone.
The Syrian Observatory for Human Rights said in a statement on Saturday that Russian planes pounded rebel-held towns in Daraa for the first time since the summer of 2017.
The Britain-based war monitor said at least 26 Russian air raids hit the rebel zones as Syrian regime forces shelled the area, killing at least one person.
Local activists told opposition news website Zaman al-Wasl that the strikes have destroyed homes and caused "massive destruction".
Southern Syria is a strategic prize for local and global players involved in the country's convoluted seven-year war.
After securing the capital Damascus, Syrian President Bashar al-Assad appears keen to recapture the southern provinces of Daraa and Sweida, still mostly held by rebels.
Assad has sent military reinforcements there for weeks, dropped flyers demanding rebels surrender, and escalated bombardment in recent days.
Russia, the United States, and Jordan agreed in July of last year on a de-escalation zone in rebel-controlled parts of southern Syria that would tamp down hostilities there.
Since then, Moscow's warplanes - active in Syria since 2015 - had refrained from bombing rebel positions in the south.
The US has reportedly informed Syrian rebels it would not intervene.
Violence has been ratcheting up this week as Syrian government forces look to retake the south militarily.
Forces loyal to Assad began ramping up their air strikes and artillery fire on the zone on Tuesday.
At least 19 civilians in rebel-held zones have died since then, according to the SOHR.
Some 12,000 people have been displaced from Daraa province in recent days, the SOHR said, with many seeking refuge in poorly-equipped displacement camps further west.
The United Nations has warned that growing violence is putting the lives of 750,000 people in rebel parts of the south in danger.
Several civilians have also been killed in opposition fire on government zones, with regime news agency SANA reporting on Saturday that two civilians were killed in Daraa city in rebel shelling.