Syria's regime and its ally Russia resumed air strikes on the south of the country on Wednesday after rebels said talks over a government takeover had "failed".
Seven years into Syria's devastating civil war, President Bashar Al Assad has set his sights on retaking the southern region considered to be the cradle of the uprising against him.
Russia has been backing a two-week offensive by Mr Assad's forces against rebels in the southern provinces of Deraa and Quneitra, bordering Jordan and the Israeli-occupied Golan Heights.
But it is simultaneously brokering talks with rebel towns for negotiated surrenders in a carrot-and-stick strategy that Russia and the regime have used in the past to retake large areas of rebel-held territory.
More than 30 towns have already agreed to return to regime control, and talks in recent days have focused on remaining rebel territory in Deraa province's western countryside and the southern half of the provincial capital.
On Wednesday afternoon, rebels met with a Russian delegation to deliver their decision on Moscow's proposal for a regime takeover of the rest of the south.
About 90 minutes after the meeting was set to begin, the joint rebel command for the south announced the talks had "failed".
"Negotiations with the Russian enemy in Busra Al Sham have failed, after they insisted on the surrender of heavy weapons," it said in an online statement.
"No negotiations under threat," it later added on Twitter, demanding talks "under the auspices of the United Nations".
Spokesman Ibrahim Jabbawi said the talks had not produced "any results" because Moscow had insisted rebels hand over their heavy arms in one go.
"The session ended. No future meetings have been set," Mr Jabbawi said.
After the talks collapsed, Russian and regime aircraft resumed strikes on several parts of the province for the first time in four days, the Britain-based Syrian Observatory for Human Rights monitor said.
A source close to the talks said rebels would be willing to hand over their heavy weapons in several phases.
The meeting followed an hours-long session on Tuesday, in which rebels proposed that the army withdraw from recaptured towns, and grant fighters and civilians unwilling to live under regime control safe passage to rebel-held territory elsewhere.
Moscow had roundly rejected the terms and responded with a counter-proposal, the source said.
It told negotiators that population transfers were not on the table in the south, although it had agreed to them in other areas such as Eastern Ghouta and Aleppo.
Russia insisted the army would return to its pre-2011 positions and local police would take over towns in co-ordination with Russian military police.
The source had said before Wednesday's meeting that the rebels were expected to give their "final answer".
"Today will be the last round – either the rebels agree to these terms or the military operations resume," the source said.
Moscow has used tough deadlines in the past with rebels but has sometimes extended them.
That blend of military pressure and negotiated surrenders has expanded the regime's control of Deraa province to about 60 per cent – double what it held when it began operations on June 19.
The violence has displaced between 270,000 and 330,000 people, according to the UN, many south to the border with Jordan or west to near the Israeli-occupied Golan Heights.
Both countries have kept their borders closed, despite mounting calls by rights groups to let Syrians escape to safety.
On Wednesday, Human Rights Watch demanded both Jordan and Israel allow asylum seekers in.
"The abject refusal by Jordanian authorities to allow asylum seekers to seek protection not only goes against their international legal obligations, but against basic human decency," said HRW's Lama Fakih.
Some displaced families whose hometowns had fallen back under regime control have returned, but even that journey holds risks.
Eleven members of a single family were killed overnight in a landmine blast as they returned to Al Mseifra, which had "reconciled" with the government, the Syrian Observatory for Human Rights said on Wednesday.
More than 140 civilians have died since the assault began.
World powers have criticised the operation for violating a ceasefire announced last year by Washington, Amman and Moscow, but that has not halted the blitz.
The UN Security Council on Thursday will hold a closed-door emergency meeting on the offensive.
Residents and displaced Daraa natives gathered in front of UN offices in a rebel-held town in Quneitra province to protest global inaction.
They "ask the United Nations and the world for protection and international guarantees" for their lives, said opposition official Ali Salhadi.