At least 43 people have been killed in a series of air strikes believed to have been carried out by Russian planes in the Syrian city of Idlib, according to activists and residents.
A marketplace, homes and official buildings were all hit, reports say. Bodies were still being pulled from the rubble, a civil defence worker said.
Russia has not confirmed whether it carried out strikes in the area.
Russia began an air campaign to bolster President Bashar al-Assad in September.
It says it has targeted only "terrorists", above all jihadist militants from the Islamic State (IS) group, but activists say its strikes have mainly hit Western-backed rebel groups.
The Local Co-ordination Committees, a network of anti-regime activists, said that more than 170 people had been injured in the strikes.
A coalition of Islamist anti-government rebels captured the city of Idlib earlier this year.
It is only the second provincial centre to fall into rebel hands during the conflict, after Raqqa was seized by IS.
Their advances in the area pose a threat to the nearby coastal region, which is a bastion of support for the government of President Assad.
Also on Sunday, an explosion on a military bus on the outskirts of the capital Damascus injured several people.
Earlier this week, the UN Security Council unanimously adopted a resolution outlining a peace process in Syria.
The resolution endorsed talks between the Syrian government and opposition in early January, as well as a ceasefire, but disagreements remain between world powers over Mr Assad's role in Syria's future.
The Syrian war, which is heading towards its fifth year, has killed more than 250,000 people and displaced millions more, the UN says.