By Suleiman Al-Khalidi and Angus McDowall
AMMAN/BEIRUT (Reuters) - A string of Syrian rebel-held towns and villages accepted government rule on Saturday as insurgent lines collapsed in parts of the southwest under an intense bombardment that the United Nations says has forced 160,000 people to flee.
The southwest was an early hotbed of the uprising against President Bashar al-Assad and defeat there would leave rebels with just one remaining stronghold -- the area around Idlib province bordering Turkey in the northwest.
Rebels met Russian negotiators on Saturday to seek peace terms for Deraa province, where most of their southwest territory is located, but said these failed. Moscow is Assad's strongest ally and its air power since 2015 has been crucial to his recapture of vast swathes of Syria.
Local groups in many towns seized by the army in recent days had negotiated their own surrender deals independently of the main rebel operations rooms after heavy air raids.
State television broadcast footage from inside the towns of Dael and al-Ghariya al-Gharbiya, where people were shown chanting pro-Assad slogans. A war monitor and a military media unit run by the government's ally Hezbollah said numerous other towns and villages had agreed to come back under Assad's rule.
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Fierce battles were still roiling the area around Deraa city, near the Jordanian border, where the army was trying to capture a disused air base, rebels said, and the northwestern chunk of Deraa province remains in opposition hands.
Air raids meanwhile intensified, said the monitor, the Syrian Observatory for Human Rights, as displaced people flocked to the border areas least likely to be hit and the United Nations warned about a humanitarian catastrophe.
After the peace talks failed on Saturday, warplanes launched a new wave of strikes on the rebel-held towns of Bosra al-Sham, al-Nuaima and other areas, the Observatory reported, causing deaths, injuries and damage.
The army's offensive follows the capitulation of rebel enclaves near Homs and Damascus, including eastern Ghouta, which was recaptured after a scorched-earth assault that killed over a thousand civilians and laid waste to several towns.
Warfare in the southwest could risk a further escalation because of its proximity to Israel. The Israelis have already targeted Iran-backed militias fighting on Assad's side, which they have vowed to keep far from their country's borders.
The government's offensive so far has focused on Deraa province, which borders Jordan, but not Quneitra province abutting the Israeli-occupied Golan Heights.
The entire southwest is part of a "de-escalation zone" agreed last year by Russia, the United States and Jordan. Despite Washington's threats that it would respond to breaches of that arrangement, it has shown no sign of doing so, and the opposition's top negotiator on Thursday accused it of having struck a "malicious deal" to stay silent.
Insurgent negotiators had held meetings with Russia to try to agree a deal for all of Deraa province to come back under government sovereignty, but without the army or police entering the area.
The talks in the town of Bosra al-Sham, whose Roman citadel is listed as a UNESCO world heritage site, collapsed on Saturday, however, as the insurgents rejected Russia's proposed terms for their surrender, a rebel spokesman said.
Jordan has been facilitating talks between rebel factions and Moscow over a deal that would end the violence in exchange for the return of state rule in Deraa province on its border.
Russian negotiators have demanded rebels accept terms like those agreed for eastern Ghouta, where insurgents either left for opposition territory in the northwest along with their families or accepted the return of state rule, Jabawi said.
The southwest rebels did not accept this, and were instead proposing the return of civilian state institutions in the opposition areas and the entry of Russian military police rather than Syrian government forces.
The army has already captured large parts of the eastern zone of rebel-held territory in Deraa province less than two weeks into its offensive, however.
In eastern parts of Deraa, the army took control of al-Ghariya al-Sharqiya and al-Ghariya al-Gharbiya and Um Walad, after taking al-Harak, Busra al-Harir and surrounding areas in recent days.
In western parts of Deraa, the army has regained control over the towns of Dael and Ibta. There were contradictory reports over whether it had also gained the areas of al-Musayfra and Kuheil in eastern Deraa, and Tafas, al-Muzairib and Sheikh Saad in western Deraa.
The Observatory reported that warplanes carried out 32 air strikes overnight as the offensive continued, hitting nine towns in Deraa province. So far, about 100 civilians have been killed in air raids and shelling since June 19, it said.
(Reporting by Suleiman Al-Khalidi and Angus McDowall, writing by Angus McDowall, Editing by Catherine Evans)