Talks in the town of Bosra al-Sham on Saturday broke down as the army seized more ground in its offensive, with insurgent lines in some areas collapsing, and a string of towns and villages accepting the return of government rule after intense bombardment.
Fighting and bombardment calmed overnight, the Syrian Observatory for Human Rights said, but reignited on Sunday around Tafas, northwest of Deraa, along with heavy air strikes.
Syrian President Bashar al-Assad's offensive in the southwest aims to reclaim one of two remaining rebel strongholds in Syria, the other being Idlib and adjacent areas in the northwest. Assad's forces captured the last enclaves near Damascus and Homs earlier this year.
Southwest Syria is a "de-escalation zone" of reduced warfare and bombardment agreed by Russia, Jordan and the United States last year. Washington warned it would respond to violations of this agreement, but has done nothing so far. Last week, rebels said the United States had told them not to expect any American military support.
The opposition's chief negotiator in wider UN peace talks, Nasr al-Hariri, last week accused the United States of complicity in Assad's southwest offensive, saying American silence could only be explained by "a malicious deal."
Peace talks in the town of Bosra al-Sham, home to a UNESCO world heritage site, failed on Saturday when insurgents rejected Russian terms for their surrender, but began again on Sunday under the auspices of Jordan, rebel spokesman Ibrahim al-Jabawi said.
Jordan's Foreign Minister Ayman Safadi said the kingdom was engaged in intensive diplomacy with all parties in the conflict to help broker a ceasefire that would ease plight of displaced civilians.
"We are moving in all directions and with all the parties to bring a ceasefire and protect civilians," he said in a Tweet on Saturday.
Air strikes have pounded the region since the offensive ramped up two weeks ago, causing at least 160,000 people to flee their homes according to the United Nations.
On Saturday at least 10 civilians were killed when bombs were dropped on the rebel-held village of Ghasam, relief workers said. The Observatory says more than 100 civilians have been killed since an escalation in fighting on June 19.