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 of rebel neighbourhoods was the most intensive since the launch of the Russian-backed offensive on June 19.

The Syrian Observatory for Human Rights, a Britain-based monitor, said hundreds of missiles and crude barrel bombs were unleashed by Syrian and Russian aircraft overnight.

Observatory director Rami Abdel Rahman said the bombardment was an "attempt to make the rebels back down after they refused to subject themselves to the Russian terms for a ceasefire."

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 a grove of olive trees, told AFP.

- 'Afraid of everything' -

"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 Observatory said air strikes were ongoing, including on the town of Tafas in the northwest of Daraa province and on towns and villages near the Jordanian border.

Six civilians, including a woman and four children, were killed in the town of Saida, which regime ground forces were trying to take, it said.

The intensity of the bombardment allowed government forces to seize control of a security checkpoint on the Jordanian border for the first time in more than three years, it said.

The intensified assault came after rebels said Wednesday that talks brokered by regime ally Russia on ending the offensive had failed.

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 Russia and the regime have used to retake swathes of territory including the Eastern Ghouta region near Damascus earlier this year.

More than 30 towns in the south have already agreed to surrender, expanding the regime's control of Daraa province to around 60 percent, double what it held before the start of the offensive.

- Failed negotiations -

The talks this week focused on remaining rebel territory in Daraa province's western countryside and the southern half of the provincial capital.

After meeting with a Russian delegation on Wednesday afternoon, the joint rebel command for the south said talks had failed over the issue of the rebels surrendering their heavy weapons.

Rebel sources said Moscow had demanded rebels hand over all their heavy arms in one go but that opposition fighters wanted to do so in several phases.

Moscow has 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.

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

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

The southern 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 by human rights groups 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.

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