A war monitor reported Syrian military helicopters dropped barrel bombs on rebel-held areas of the southwest 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 had dropped more than 12 barrel bombs on rebel-held territory northeast of Deraa, causing damage but no deaths.
President Bashar al-Assad has sworn to recapture the area bordering Jordan and the Israeli-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.
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 Assad's Russian allies last year to contain the war in the southwest.
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 Assad's seven-year 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 southwest 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.
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.