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.
Earlier this week, a suspected chemical attack hit the town of Khan Sheikhun in Idlib, killing at least 87 civilians, including 31 children.
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Much of the international community accused Assad's government of responsibility for that attack, though Damascus denied involvement.
The Khan Sheikhun 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.