Death toll may rise in the attack in which over a dozen people were also wounded, Syrian Civil Defence says.
Russian air strikes killed at least five civilians and wounded 15 others in Syria's opposition-held Idlib province on Tuesday, witnesses said.
The death toll may rise in the province – one of the de-escalation zones where attacks are prohibited, the Syrian Civil Defence said.
Mustafa Haj Yusuf, head of the Syrian Civil Defence, also known as the White Helmets, told Anadolu Agency that the air attacks came early on Tuesday in residential areas of Idlib.
The White Helmets work in opposition-held areas and have rescued thousands of civilians caught up in the fighting.
The attacks mark a continued violation of the UN's ceasefire, he added.
According to Yusuf, warplanes hit the central area of Idlib, and other areas like Binnish, Maarrat Misrin, and Ram Hamdan.
Idlib is the biggest region of Syria still held by rebels fighting the Syrian regime.
Idlib city under attack again, non-stop airstrikes in and around the city of #Idlib and news about killed civilians in #Taftanaz city. @SyriaCivilDefe teams working to respond quickly to the locations of the raids. pic.twitter.com/QPPmB9KERu— The White Helmets (@SyriaCivilDef) March 12, 2018
Syria has been locked in a devastating conflict since early 2011 when Bashar al Assad's regime cracked down on demonstrators with unexpected ferocity.
According to UN officials, to date hundreds of thousands of people have been killed in the conflict and millions displaced.
Britain-based war monitor Syrian Observatory for Human Rights says about 511,000 people have been killed in the conflict so far.
It says it has the names of more than 350,000 of them.