The Syrian military and its Russian allies pummeled rebel-held towns in southwest Syria overnight with more than 600 airstrikes in 15 hours, the Syrian Observatory for Human Rights war monitor said.
After four days of scaled back bombardment, the intense airstrikes resumed on Wednesday following the collapse of talks between insurgent groups and Russian officers.
Syrian President Bashar Assad and his allies are fighting to recapture the southwest, one of the last remaining rebel strongholds in Syria, along with a region of the northwest bordering Turkey.
His two-week offensive, backed by Russian air power, has taken a large chunk of rebel territory northeast of the provincial capital of Daraa, as a string of towns surrendered.
The fighting and airstrikes have already driven more than 320,ooo people in southwest Syria from their homes, the United Nations said Wednesday, seeking shelter along the frontiers with Jordan and Israel.
Both countries have said they will not open their borders to refugees – Jordan took in more than half a million earlier in the war – but both have distributed supplies inside Syria.
The airstrikes targeted the towns of Tafas, northwest of the provincial capital Daraa, and Saida to its east, the Syrian Observatory for Human Rights said.
Russian airstrikes paused on Saturday evening, the Observatory said, as rebels negotiated with Russia over a deal to end the fighting by accepting the return of state sovereignty, but they have not been able to agree on the terms.
"The talks with the Russian enemy in Bosra al-Sham have failed because of their insistence on handing over heavy weapons," Abu Shaima, a spokesman of the central operations room representing the main Free Syrian Army factions negotiating with the Russians said.
The opposition had also demanded safe passage to opposition-held northern Syria, a request denied by Russia, according to other activists.
The talks' failure comes after a relative lull in the fighting that has raged for two weeks in the south. The government has already regained control of most of Daraa province.
Another rebel spokesman, Ibrahim al-Jabawi, said the talks did not reach any conclusion.
"Russia wanted heavy weapons handed over in one go. The opposition wanted to surrender them in stages after tens of thousands of displaced people returned home," he said.
The first round of talks on Saturday prompted rebels to walk out, saying Russian terms amounted to a humiliating surrender. Jordan persuaded them to return to the negotiating table, official sources said.
Numerous towns in the southwest have already struck their own deals to surrender to the government amid military advances and aerial bombardment, independently of the main rebel factions.
Warfare in southwest Syria is sensitive to neighboring Jordan and Israel. The area is also a "de-escalation zone," as agreed by Jordan, the United States and Russia last year to reduce fighting.
Although Washington warned it would respond to violations of the agreement, it has yet to act. Rebels have said the United States told them to expect no American military assistance.
Jordan's Foreign Minister Ayman Safadi met his Russian counterpart Sergey Lavrov in Moscow earlier Wednesday, and warned of a possible humanitarian catastrophe in southern Syria if no cease-fire is reached.
Lavrov said all Syria-related issues were expected to be discussed at an upcoming meeting between U.S. President Donald Trump and Russian President Vladimir Putin.
