BEIRUT (Reuters) - 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.

Forces loyal to Syria's President Bashar al-Assad gesture in the town of Busra al-Harir, northeast of Deraa, Syria in this handout released on June 28, 2018. SANA/Handout via REUTERS

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.”

Vassily Nebenzia, Russia’s ambassador to the United Nations, indicated that Russia would no longer uphold the de-escalation zone, saying it was among the last strongholds of al Qaeda’s Syrian branch and Islamic State, and the Syrian army has a “legitimate right” to fight “terrorists.”

“Every de-escalation zone that we established was not established for good,” Nebenzia told a news conference at the United Nations.

“I hope that all of the de-escalation zones will go into history and will have Syria territorially united under the control of the legitimate government. The de-escalation zone for those who are ready to uphold de-escalation still exists. But for those who resist any de-escalation, it doesn’t.”

Assad’s offensive in the southwest has been backed by air strikes and shelling that have killed 96 civilians since June 19, the Syrian Observatory for Human Rights war monitor said, including 49 on Wednesday and Thursday. Some 67 pro-government forces and 54 rebels have also been killed.

Insurgent territory in the southwest is strung along the borders with Jordan and the Israeli-occupied Golan Heights, narrowing to only a few kilometers 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 center 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.

“SILENCE”

Warfare in southwestern Syria is sensitive to neighboring 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.

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Israel wants Iranian forces and Tehran-backed militia kept far from its border.

A U.S. State Department official said on Wednesday Russia had agreed to work with Damascus to remove such forces a defined distance from the Israeli and Jordanian borders, and from opposition-held territory in the southwest. “However, we have not yet seen this take place,” the official said.

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.

The United Nations has said the southwestern assault has uprooted at least 50,000 civilians.