The UK-based Syrian Observatory for Human Rights reported that missiles had hit a public square in the rebel-held village of al-Janudiya.
Many people had gathered there to go shopping, the group added.
Al-Janudiya is situated in the west of Idlib province, which is now almost completely controlled by rebel forces.
An alliance including al-Qaeda's affiliate in Syria, the al-Nusra Front, seized control of the provincial capital at the end of March, and the major town of Jisr al-Shughour, near al-Janudiya, a month later.
The rebels are now advancing on the Mediterranean coastal province of Latakia, a stronghold of President Bashar al-Assad and his Alawite sect.
The Syrian Observatory and the Local Co-ordination Committees (LCC), an opposition activist network, both said government aircraft had attacked al-Janudiya on Monday.
The LCC put the death toll at 60 and warned that it was likely to rise because some of the dozens of wounded people were in a critical condition.
Syrian government officials have so far not commented on the reports.
The LCC also reported that several people had been killed on Monday in a government air strike in the town of Taftanaz, in eastern Idlib, and that four others had died when government helicopters dropped barrel bombs in the town of Tal Rifaat, in neighbouring Aleppo province.
The UN says more than 220,000 people have been killed since the uprising against Mr Assad began four years ago. Almost 12 million others have been displaced.
At least 49 civilians, including six children, have been killed in air strikes by government forces in north-western Syria, activists say.