A Syrian army air strike killed 49 people in Idlib province in northwest Syria on Monday, the Syrian Observatory for Human Rights reported.
The Britain-based monitoring group, which gathers information from sources inside Syria, said the raid was on the town of al-Janudiyah and the dead included six children and four women.
Syrian officials could not immediately be reached for comment. The Observatory said the death toll would probably increase. Reuters could not independently verify the report.
Insurgent groups have been making steady gains in Idlib province since late March, when they captured the provincial capital from government control.
The advances have put President Bashar al-Assad under more military pressure than ever before in the four-year-old war, in which an estimated 220,000 people have died.