BEIRUT: The Syrian army battled rebels in an important town in southwest Syria on Thursday, a media unit run by its ally Hezbollah and a war monitor said, as intensifying air strikes killed dozens of people in the area.
President Bashar al-Assad has sworn to take back every inch of Syria, and recapturing the southwest, one of the first hotbeds of the uprising against him, would leave rebels with only one remaining stronghold, in the northwest.
The area is in a "de-escalation zone" agreed last year by the United States, Jordan and Assad's ally Russia to curb fighting. But despite American warnings that it would respond to an attack, it has not done so and Syrian opposition figures on Wednesday decried Washington's "silence".
Assad's offensive in the southwest has been backed by air strikes and shelling that have killed 93 civilians since June 19, the Syrian Observatory for Human Rights war monitor said, including 46 on Wednesday and Thursday.
Insurgent territory in the southwest is strung along the borders with Jordan and the Israeli-occupied Golan Heights, narrowing to only a few kilometres wide at the city of Deraa.
The fighting so far had mostly focused on areas northeast of Deraa, where the army and allied militia recaptured a string of villages, but was extended to the city's outskirts on Tuesday.
The Hezbollah media unit said the army had captured the town of al-Harak, northeast of Deraa. The Observatory said it had advanced into the town's centre but fighting continued.
A rebel official said the army and allied militia were also seeking to cut insurgent ground in the southwest in two by storming an air base close to the Jordanian border.
"The goal for them is to split the western Deraa countryside from the city and the eastern Deraa countryside. Praise God, so far the fighters are standing strong and the regime was not able to advance," said Abu Shaima, a rebel spokesman.
The Britain-based monitoring group and two rebel sources said aircraft had bombed Busra al-Sham, Nawa, Rakham and other towns in the province.
Warfare in southwestern Syria is sensitive to neighbouring Jordan and Israel, though government bombardments so far have not focused on territory nearest to the Golan Heights.
In Riyadh, the chief Syrian opposition negotiator Nasr al-Hariri condemned "U.S. silence" over the offensive in the de-escalation zone, comparing it to Washington's prompt use of force against attacks on forces it is allied to elsewhere.
Only a "malicious deal" could explain the lack of a U.S. response to the assault, Hariri said.
France said it was extremely concerned by the attack, which it warned could carry the risk of regional destabilisation.
There was no immediate comment on civilian deaths from the air strikes from Damascus or Moscow, which have said they only target armed militants in the seven-year conflict.
"We can't even catch up to count the air strikes," said Abdallah Mahameed, a rebel official in Deraa. "The house is shaking around us."
Abu Jihad, a local opposition official near the border, said the violence since Wednesday had forced at least 11,000 people to flee their homes and towns. If the escalation continues, he said, people would head closer to the frontiers with Jordan and the Golan.
A Jordanian official who requested anonymity said the Jordanian army had stepped up its state of readiness at the border. The country already hosts more than 650,000 registered Syrian refugees and said it will not open the border for more.
Syrian state television said the air force targeted militants in Busra al-Sham in east Deraa province on Thursday, and that government forces entered two villages further northeast.
It said 450 insurgents in the Laja area, which the army seized earlier this week, handed themselves in with their weapons to "settle their affairs" with the government.
Assad's military, with the help of Russian air power and Iranian-backed militias, has driven rebels out of all their enclaves near the capital Damascus this year.
The United Nations says the southwestern assault has uprooted at least 45,000 civilians.
(Reporting by Ellen Francis in Beirut and Suleiman al-Khalidi in Amman; additional reporting by Sarah Dadouch in Riyadh and John Irish in Paris; writing by Angus McDowall; editing by Mark Heinrich)