Several opposition officials and commanders later accused rebel fighters from Shabab a-Sunnah, a formerly western-backed rebel faction based in southern Syria's Daraa province, of agreeing to an initial surrender deal with the Syrian government during Sunday's talks-a claim the group's leadership fiercely denies.
With the deals in Daraa, the regime has further chipped away at a U-shaped patch of territory controlled by the rebels in southern Syria.
UN High Commissioner for Human Rights Zeid Raad Al Hussein warned that many civilians risk being trapped between government forces, rebels, and ISIS, which has a small foothold there - an outcome he said would be a "catastrophe".
Jordan stepped in to try to avert further violence and stem another wave of displacement across its border with Syria on Sunday, mediating new talks between rebels and the Syrian government's main ally Russian Federation for a truce in the southwest.
Airstrikes pounded rebel-held areas in southwestern Syria on Thursday, killing at least 17 civilians in an underground shelter and driving thousands from their homes, as scores of displaced people protested near the Israeli-occupied Golan Heights, demanding worldwide protection.
A team representing the Free Syrian Army's (FSA) rebels was now holding talks with Russian officers in the town of Busra al Sham, Ibrahim al Jabawi, the spokesman for the central operations room representing FSA negotiators told Reuters.
But just under half of Daraa province - including part of the provincial capital of the same name - still remains in the hands of rebels, and the United Nations is putting the numbers of internally displaced at over 160,000.
The Observatory said at least 46 civilians, including 15 children, have been killed since the offensive began, including the victims of Thursday's airstrike. The army had gained control over the towns of al-Harak, Ibta and Rakham, it said, and a rebel said opposition lines in one area had collapsed.
Ibrahim Jabawi clarified in his statement following the failed talks with Russian Federation that rebel territory next to the Quneitra province, bordering the Golan Heights that is held by Israel, is not part of their discussions.
Daraa Governorate, which is located at the intersection of Jordan, the occupied Golan region and Lebanon, was declared a de-escalation zone in July 2017, when a trilateral ceasefire agreement was reached, with the US, Russia and Jordan as guarantors. Jordan, which is already hosting 660,000 registered refugees, says it can not accept any more and has sealed its border, despite appeals from aid groups.
The war has been going Mr Assad's way since Russian Federation intervened on his side in 2015, when he held just a fraction of the country.
Israel has refused to accept refugees fleeing the more than seven-year conflict in Syria, a country with which it remains officially in a state of war.
"There are possibilities for doing a larger negotiation on helping to get Iranian forces out of Syria and back into Iran, which would be a significant step forward", he said.