The Syrian Observatory for Human Rights says it believed the aircraft targeting Mashara were Russian jets striking from high altitudes
Rebels and a UK-based activist group have reported fighter jets bombing an opposition-held town in southern Syria's Quneitra province, in the first such aerial strike in a year.
The Syrian Observatory for Human Rights said it believed the jets that targeted Mashara were Russian, striking the town from high altitudes, Reuters news agency reported on Sunday.
Another rebel source said artillery from nearby Syrian army barracks launched a salvo of missiles on the town.
"[Sunday's] bombardment started from 3am (midnight GMT) and continued for the last six hours," said Rami Abdel Rahman, the head of the UK-based activist group.
"Around 800 missiles and shells have been fired against several areas in the province and fierce fighting continues on the ground between regime forces and rebels," the group's director said.
Regime forces have retaken the area of Mashara, Abdel Rahman added.
"Bombardments have also hit areas in [neighbouring] Daraa province," he said. The director did not immediately provide an estimate of casualties.
After securing Damascus and outlying areas of the capital, President Bashar al-Assad's forces on 19 June launched an offensive against Daraa, with support from Russia.
The Daraa onslaught took less than three weeks to break the rebels' grip, with the regime now in control of more than four-fifths of that province's territory, the Observatory said.
Rebels and armed groups still control 70 percent of Quneitra, while the Syrian government is in full control of Sweida, a third southern province, the Observatory added.
Quneitra province borders the Israeli-occupied Golan Heights.
Meanwhile, Syrian rebels and their relatives began evacuating Daraa on Sunday, the Observatory and a correspondent from the AFP news agency said, more than seven years after the country's uprising erupted there.
The transfers were part of a ceasefire deal brokered by government ally Russia, and came after more than two weeks of ferocious bombardment of Daraa city and the surrounding province of the same name.
On Sunday, hundreds of fighters and some of their relatives, carrying suitcases packed with clothes, boarded around 15 buses in Daraa city, AFP's correspondent there said.
The vehicles were parked on a main road connecting the city's government-held north with its rebel-held south.
The AFP correspondent said the buses were searched by Russian military forces before setting off just after midday for the rebel-held northwestern province of Idlib.
The Observatory said an estimated 1,400 people were expected to be evacuated in a single group on Sunday.
They included rebels from the broader province, said Abdel Rahman.
"Buses began moving from the gathering point towards the edge of the city to be searched," he told AFP.
Moscow and Damascus reached a deal with rebels in early July for all of Daraa province, then agreed on terms for the city on Wednesday.
Rebels began handing over their heavy weapons on Saturday and continued to do so on Sunday morning.
Boosted by Russia's entry into Syria's civil war in 2015, Assad's regime now controls 61 percent of the country's territory, according to the Observatory.
Syria's conflict has killed more than 350,000 people since March 2011, although the UN has long stopped keeping count of casualties in the seven-year conflict.