Beirut - Waves of air strikes pounded rebel-held areas of southern Syria on Thursday as pro-regime forces launched their most intensive bombing campaign yet in a two-week-old offensive.

The renewed assault came after the failure on Wednesday of Russian-brokered talks to end the offensive in Daraa province, which has killed dozens and forced tens of thousands from their homes.

An AFP correspondent on the edge of the rebel-held south of the city of Daraa, the divided provincial capital, said the bombing was the most intensive since the launch of the Russian-backed offensive on June 19.

“From last night until now, Russian airplanes are pursuing a scorched earth policy,” said Hussein Abazeed, spokesman for the joint rebel command for the south. “This is to force rebels to return to negotiations.”

Hundreds of missiles and crude barrel bombs were unleashed by Syrian and Russian aircraft overnight, the Syrian Observatory for Human Rights said.

Samer Homssi, who had fled to the outskirts of Daraa with his wife and four children, described a fierce onslaught of regime air strikes and shelling.

“The bombardment has not stopped for one moment since the announcement that the negotiations failed,” the 47-year-old, whose family was taking shelter in an olive grove, told AFP.

“We are living here in the olive trees, afraid of everything, the shelling, the insects, with no water to drink or any medical services nearby. The situation is very difficult.” The Britain-based Observatory said air strikes were ongoing, including on the town of Tafas in the northwest of Daraa province and on areas near the Jordanian border.

Six civilians, including a woman and four children, were killed in the town of Saida.

Hours later, regime forces fully retook the town and also seized control of a security checkpoint on the Jordanian border for the first time in more than three years, the monitoring group said.

State television showed footage of columns of smoke and said air strikes were aiming to cut rebel access routes in the southernmost parts of the province.

The regime’s main prize, the Nasib border crossing, remains in rebel hands. After retaking large parts of the country since Russia intervened in Syria in 2015, regime forces have set their sights on the southern provinces of Daraa and Quneitra, bordering Jordan and the Israeli-occupied Golan Heights.

Moscow has been brokering talks with rebel towns for negotiated surrenders in a carrot-and-stick strategy that it and the regime have used to retake swathes of territory including the Eastern Ghouta region near Damascus earlier this year.

More than 30 southern towns have already agreed to surrender, expanding the regime’s control of Daraa province to more than 60 percent, double what it held before the offensive. The talks this week focused on remaining rebel territory in Daraa province’s western countryside and the southern half of the provincial capital. At a meeting between rebel and Russian negotiatiors on Wednesday, Moscow insisted opposition factions hand over their heavy weapons in one go, while rebels wanted to do so in several phases.

Moscow also reportedly refused requests from some rebels for safe passage to opposition-held territory in other parts of Syria, as was done in Eastern Ghouta and Aleppo. The rebel spokesman for the south, Abazeed, told AFP that during Wednesday’s meeting a Russian negotiator who identified himself as Alexander had threatened renewed strikes from Russian’s airbase in northwestern Syria.

“He threatened that if we didn’t agree to the deal, 40 planes would leave Hmeimim airport to bomb the south.” In a statement on Thursday, the joint rebel command said it wanted “real guarantees” and UN supervision of future talks.

“Russian warplanes are translating the threatening tone of the negotiations into bombs, fire, and destruction in southern Syria,” it said.

Daraa is considered the cradle of the 2011 uprising against President Bashar al-Assad that triggered Syria’s devastating civil war.

Nearly 150 civilians have died since the assault in the south began, according to the Observatory.

The offensive has also displaced between 270,000 and 330,000 people, according to the United Nations, 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 to let Syrians escape to safety.

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.

UN High Commissioner for Refugees Filippo Grandi said that the hostilities were hindering UN efforts to provide cross-border aid from Jordan.

“Thousands of innocent lives are going to be lost, once again, if urgent action is not taken,” he warned.

The UN Security Council will hold a closed-door emergency meeting on the offensive on Thursday.