Air strikes on a market and residential areas in opposition-held north-western Syria have killed at least 31 civilians, rescuers and activists say.
Fifty other people were injured when warplanes targeted the town of Maarat al-Numan, in southern Idlib province, causing widespread destruction.
Russia has denied reports that its aircraft carried out the strikes.
The Russians back Syria's government, which is trying to recapture the region from rebels and jihadists.
The UN said last week that more than 350 civilians had been killed and 330,000 forced to flee their homes since fighting escalated on 29 April.
Idlib, northern Hama and western Aleppo province make up the last opposition stronghold in Syria after eight years of civil war. It is supposedly covered by a truce brokered in September by Russia and opposition-backer Turkey that spared the 2.7 million civilians living there from a major government offensive.
The Syria Civil Defence, whose volunteer first responders are widely known as the White Helmets, says Maarat al-Numan was bombed at least eight times by Russian warplanes on Monday morning.
An AFP news agency photographer who witnessed the aftermath said he had seen casualties drenched in blood being led away by rescuers and residents.
"Bodies are lying on the streets. May God take revenge on [Russian President Vladimir] Putin and [Syrian President Bashar al-Assad] for their crimes," Abdul Rahman al-Yasser, one of the rescuers, told Reuters news agency.
The Syria Civil Defence said one of its volunteers had been killed in a "double tap" attack, when an initial strike is followed by a second, deliberately timed to hit those gathered at the scene.
Russia's defence ministry dismissed as "fake" the allegation that it carried out the strikes, saying its warplanes had not flown any missions over Idlib on Monday.
Air strikes killed another six people in the town of Saraqeb on Monday, according to the Syrian Observatory for Human Rights, a UK-based monitoring group.
Syria's state news agency meanwhile reported that seven civilians had been killed by "terrorist rocket attacks" in neighbouring Hama province.
Civilian areas have been hit frequently since the Syrian government and Russia intensified their attacks almost three months ago in response to what they said were repeated truce violations by jihadists linked to al-Qaeda who dominate the opposition stronghold.
Russia and the Syrian government have denied carrying out unlawful attacks.
A spokesman for the UN Office for the Co-ordination of Humanitarian Affairs, David Swanson, told AFP that Idlib had "fast become one of most dangerous places in the world for civilians and aid workers today; a crisis within a crisis".
"This violence must stop and it must stop now," he added.