Waves of airstrikes pounded opposition-held areas of southern Syria on Thursday (July 5th) as Syrian regime and allied forces launched their most intensive bombing campaign since the offensive began on June 19th.

The renewed assault came after the failure on Wednesday of Russian-brokered talks to end the offensive in Daraa province.

Nearly 150 civilians have died since the assault began, the Observatory said, with the UN reporting that up to 330,000 people have been displaced.

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

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

Airstrikes are ongoing, the Observatory said, targeting the town of Tafas in northwest Daraa province and 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.

Regime forces also seized control of a checkpoint on the Jordanian border for the first time in more than three years, the Observatory said.

Russia-brokered talks fail

The intensified assault came after the opposition said Wednesday that talks brokered by Russia for a negotiated surrender on ending the offensive had failed.

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

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

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

Opposition sources said Russia had demanded they hand over all their heavy arms at once, but that opposition fighters wanted to do so in several phases.

Russia also reportedly refused requests from some opposition fighters for safe passage to opposition-held territory in other parts of Syria.

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