Civil defense members inspect the damage at a site hit by airstrikes on Tuesday, in the town of Khan Sheikhoun in rebel-held Idlib.

An air strike killed 18 civilians including five children in Syria’s northwestern province of Idlib on Saturday, the Syrian Observatory for Human Rights monitor said.

The monitoring group updated its earlier death toll of 15 for the strike on Urum al-Joz, saying three more people had died of serious injuries.

It said the strike was believed to have been carried out by Russian planes, which operate in support of the Syrian government.

The Observatory, which relies on a network of sources inside Syria for its information, says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.

Idlib province is controlled by a rebel alliance that includes a former Al-Qaeda affiliate, and is regularly targeted by both the Syrian government and its Russian ally.

Earlier this week, a suspected chemical attack hit the town of Khan Sheikhoun in Idlib, killing at least 87 civilians, including 31 children.

Much of the international community accused Assad’s government of responsibility for that attack, though Damascus involved responsibility.

The Khan Sheikhoun strike prompted Washington to take its first direct military action against Assad’s government, with missile strikes against a central air base on Friday morning.

More than 320,000 people have been killed in Syria since the conflict began in March 2011 with anti-government demonstrations.