Syrian government helicopters dropped barrel bombs on opposition areas of the south-west on Friday for the first time in a year, escalating an assault that has so far included artillery but only limited use of air power.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said Syrian government helicopters dropped more than 12 barrel bombs on rebel-held territory northeast of Deraa, causing damage but no deaths.
Abu Bakr Al Hassan, spokesman for the rebel group Jaish Al Thawra, said the bombs were dropped on three towns and villages, and that air strikes had hit another.
President Bashar Al Assad has sworn to recapture the area bordering Jordan and the Israel-occupied Golan Heights and the army began ramping up an assault there this week.
The attack has been concentrated on the towns of Al Harak and Busra Al Harir, which would bisect a finger of rebel ground jutting northwards into land held by the Syrian government.
Syrian state television said on Friday that army units had targeted "lairs and movements of terrorists" in the area.
A big offensive risks a wider escalation, as the United States has warned Damascus it will respond to breaches of a "de-escalation" brokered by Washington and Mr Al Assad's ally Russia last year to contain the war in the south-west.
On Thursday, the US State Department said it had reports the Syrian government had "violated the south-west de-escalation zone and initiated air strikes, artillery and rocket attacks", and warned of "serious repercussions".
The government's use of barrel bombs on Friday tests the US position, said Mr Al Hassan, the rebel spokesman, whose group fights under the banner of the Free Syrian Army (FSA).
"I believe it is testing two things: the steadfastness of the FSA fighters and the degree of US commitment to the de-escalation agreement in the south," he said.
The region is also of strategic concern to Israel, which has struck Iran-backed militia allied to the army.
Those militia, including Lebanon's Hezbollah, have played an important role in Mr Al Assad's seven-year-old war against the rebels, including since Russia entered the conflict in 2015.
The Russian ambassador to Lebanon was quoted on Friday in the pro-Hezbollah newspaper Al Akhbar as saying the Syrian military was recovering the south-west with help from Moscow.
"We say that the Syrian army now, with support from Russian forces, is recovering its land in the south and restoring the authority of the Syrian state," it quoted him as saying in an interview.
"Israel has no justification to carry out any action that obstructs the fight against terrorism," he added.
Mr Al Assad has this year recaptured the last remaining enclaves of insurgent territory near the capital Damascus and the city of Homs, including the densely populated Eastern Ghouta region.
But there are still large areas outside his control. Apart from the south-west, the rebels also hold a swathe of north-west Syria. rebel groups backed by Turkey hold parts of the northern border area.
And the quarter of Syria east of the Euphrates is controlled by an alliance of Kurdish and Arab militias supported by the United States. The US also has a base at Al Tanf, near Syria's borders with Iraq and Jordan, which controls the Damascus-Baghdad highway.
On Thursday a commander in the regional alliance backing Mr Al Assad said a US strike had killed a Syrian army officer near Al Tanf. However, the Pentagon said a US-backed Syrian rebel group had engaged "an unidentified hostile force" near Al Tanf, without casualties on either side.
The Syrian government has denied using barrel bombs, containers filled with explosive material that are dropped from helicopters and which cannot be accurately aimed. However, United Nations investigators have extensively documented its use of them during the conflict.