At least 16 people, including 13 children, were killed when Syrian air force jets bombed a town in Deraa province on Tuesday, activists say.

Those killed were attending lessons on the Koran in Ghariyah, locals told the Syrian Observatory for Human Rights, a UK-based monitoring group.

Syrian government officials have not commented on reports of the attack.

More than 230,000 Syrians have died in the civil war, which began after anti-government protests in March 2011.

Syrian rebels have been making gains in recent weeks against government forces in Deraa province, which was where unrest over President Bashar al-Assad's rule first erupted four years ago.

The town of Ghariyah lies near the eastern edge of Deraa province, not far from the location of a military base that rebel fighters seized from the regime a week ago.

The Syrian Observatory said the death toll from Tuesday's strikes was expected to rise further because a number of those injured were in serious condition.

Rebels also briefly overrun the nearby al-Thalaa air base, before government forces eventually retook it.

Earlier on Tuesday, the United States accused Syria's government of using chemical weapons in the form of chlorine-filled barrel bomb attacks.

US Secretary of State John Kerry said he was absolutely certain that what he called the "vast preponderance" of such attacks were carried out by President Assad's forces, rather than the rebels.

Kerry said the international community's patience was wearing thin with the "depravity" of the government's tactics.

President Assad has been continually accused of authorising his forces to use chemical weapons against his own people since the outbreak of the war.