Talks to end the Syrian government's offensive on Deraa province collapsed on Saturday after rebels rejected a demand to surrender by regime ally Russia.
The attack on rebel held areas by government troops and allied forces, backed by Russian air strikes, has killed more than 100 civilians forced about 160,000 to flee their homes since June 19, according to a war monitor and the United Nations.
The campaign has put more than half the province under the control of President Bashar Al Assad's government, compared with just 30 per cent earlier, according to the Syrian Observatory for Human Rights.
The UK-based war monitor said at least more eight towns surrendered on Saturday under deals brokered by Russia with local officials and fighters.
However, the collapse of talks between rebels and Russia for a deal to place the entire province under state control has dashed hopes for a quick end to the fighting.
Ibrahim Jabawi, a spokesman for the main Free Syrian Army groups, said the rebels had proposed the return of civilian state institutions in the opposition-held areas and the entry of Russian military police rather than Syrian government forces.
However, the Russian negotiators insisted on terms applied in other former rebel-held areas, where insurgents either accepted the return of state rule or left for opposition territory in the north-west along with their families.
Retaking the whole of Deraa province would be a symbolic victory for the regime, as it is seen as the cradle of the anti-Assad uprising seven years ago that spiralled into civil war.
It would also allow the reopening of the Naseeb border crossing with Jordan and the resumption of trade across the frontier.
But much of the province — including part of the provincial capital Deraa city — still remains in the hands of rebels not willing to give up the fight.
"The regime wants us to hand over everything — Deraa city, the Naseeb crossing, ourselves and the heavy weapons. It's inadmissible," a negotiator in the rebel-held part of Deraa city told Agence France-Presse.
Regime warplanes launched a new wave of strikes on rebel areas after the talks collapsed on Saturday, including in Bosra Al Sham near the Jordan border where the talks were held, the Observatory said, causing deaths, injuries and damage.
Jordan has been facilitating the talks to end the fighting, which has brought thousands of families seeking safety to its border. However, it refuses to allow in more Syrians after already hosting more than a million of their compatriots since the Syrian civil war began in 2011.
The UN chief Antonio Guterres has called for “an immediate cessation” to the military operations in Deraa.
Mr Guterres is "deeply alarmed by the military offensive in south-western Syria and its devastating toll on civilians", said a statement released by his spokesman Stephane Dujarric on Friday.
"The secretary general recalls that the south-west area of Syria is part of a de-escalation agreement agreed between Jordan, Russia and the United States.
“[He] calls on all parties to respect their obligations under international humanitarian law and human rights law, protect civilians and facilitate safe, unimpeded and sustained humanitarian access."
The UN refugee agency said on Friday that the number of displaced people in southern Syria had more than tripled to 160,000 in the latest fighting.
The agency’s Jordan spokesman Mohammed Hawar said that he expects the number to rise. The last UN figure issued on Monday was 45,000.
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".
"The real concern is that we are going to see a repetition of what we saw in Eastern Ghouta — the bloodshed, the suffering, the civilians being held, being under a siege," UN human rights spokeswoman Liz Throssell said, referring to the rebel-held region near Damascus that was captured in April after weeks of heavy bombardment that killed about 1,700 civilians.
At least five at people were killed in regime bombardment of Deraa on Saturday, the Observatory said, taking the civilian death toll in the province to 105, including 19 children, since June 19.
US President Donald Trump said on Friday that he would discuss the Syria conflict during his planned meeting with President Vladimir Putin in Helsinki next month.
"I'll talk to him about everything," Mr Trump said aboard Air Force One on his way to New Jersey.
"We're going to talk about Ukraine, we're going to be talking about Syria.”
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The chief Syrian opposition negotiator Nasr Al Hariri has decried "US silence" over the offensive and said only a "malicious deal" could explain the lack of a US response.
The war has been going Mr Assad's way since Russia intervened on his side in 2015, when he held just a fraction of the country. Today he commands the single largest part of Syria, although much of the north and east is outside his control.
The assault has so far focused on Deraa, not rebel-held parts of nearby Quneitra province in the Golan Heights which are more sensitive to Israel.
