Syrian government air strikes on opposition-held towns across the country killed at least 32 people on Monday, and wounded dozens more, a monitoring group said.

In Jassem in the southern province of Daraa, 15 civilians were killed in four air strikes, while 25 others were wounded, said the Syrian Observatory for Human Rights.

The strikes in response to a major rebel offensive that has been underway in southern Syria for months.

"As usual, the regime is striking populated areas in order to make civilian supporters of opposition fighters turn against them," Observatory director Rami Abdel Rahman said.

Rebels fighting to oust President Bashar al-Assad have suffered a spate of defeats at the hands of his forces, but they still have the upper hand in Daraa.

"Opposition fighters are making steady progress in Daraa province. The vast majority of the west of the province has completely fallen out of government control, and that is where Jassem is located," Abdel Rahman said.

Rebel fighters in the area benefit from "the fact that supply lines from Jordan are still open," he added.

The involvement of experienced fighters of Syria's Al-Qaeda affiliate, the Al-Nusra Front, has also helped the rebels to gain territory in Daraa.

Elsewhere, air strikes on Douma, the besieged rebel-held town east of Damascus, killed at least six civilians and wounded dozens more, the Observatory said.

An AFP photographer in Douma said the strikes hit residential areas and that most of the wounded were children.

At Khan Sheikhun in the northwestern province of Idlib, 11 people were killed, including a former army officer who had defected from the loyalist army to join opposition forces, said the Observatory.

The regime first deployed warplanes in the Syrian war in July 2012.

Now, nearly four years into the conflict, there are air strikes every day, despite repeated warnings from the international community that such tactics fail to discriminate between civilian and military targets.

On a separate front, Syrian Kurds fighting the Islamic State group made fresh advances near Kobane, seizing a new string of villages a week after the flashpoint town was recaptured from the IS jihadists.

"The Kurds now control an area stretching across 14 kilometres (nine miles) southwards from Kobane, 10 kilometres to the east, and 10 to 12 kilometres to the west," said Abdel Rahman.

The Observatory director also said Kurdish People's Protection Units (YPG) backed by Syrian rebels killed 10 IS members in the past 24 hours.

The Syrian conflict began as a peaceful uprising in March 2011 but escalated into a civil war after the government unleashed a brutal crackdown on dissent.

More than 200,000 people have been killed, and nearly half of the population has fled their homes.