A barrage of Russian strikes on opposition-held towns in southern Syria killed 22 civilians on Thursday (June 28th), the bloodiest day yet of the government's offensive in the strategic region, AFP reported.

With Moscow's help, Syrian President Bashar al-Assad's army has battered the south for more than a week with airstrikes, rocket fire and crude barrel bombs.

The bombardment has forced more than 45,000 to flee their homes in search of safety, according to the UN, while others huddle in their basements to wait out the raids.

On Thursday, airstrikes identified as Russian killed 22 civilians, a monitoring group said, most of them in a single town.

"At least 35 Russian airstrikes hit the town of al-Mseifra," the Syrian Observatory for Human Rights said.

"One of them hit a basement where people were taking shelter, killing 17 civilians, including five children," it said.

The hospital in al-Mseifra had been put out of service by Russian strikes on Tuesday night, the Observatory and relief organisations said.

Another five civilians were killed in other opposition-controlled areas of Daraa, the main province in southern Syria.

"This is the highest toll since the escalation began on June 19th," said Observatory head Rami Abdel Rahman, adding that it brought the overall civilian toll since then to 93.