The raid hit a residential neighbourhood in Rastan, one of the last rebel strongholds in the central province of Homs, the Syrian Observatory for Human Rights said.
The toll is likely to increase as several people are missing and others are gravely injured, the Britain-based monitor said.
Rebel groups seized Rastan in 2012 and the town has been under full siege by regime forces, who control most of the Homs province, since the beginning of this year.
The Observatory relies on a wide network of sources inside Syria to gather its information on the five-year-old conflict, which has killed more than 270,000 people and displaced millions.
It says it determines whether strikes were carried out by Syrian, Russian or US-led coalition aircraft based on the location of the raids, flight patterns and the types of planes and munitions involved.