Recent air strike on city of Maaret al-Numan in Syria's rebel-held Idlib province (AFP)
Five people were treated for "suffocation" on Sunday after Syrian government air strikes on the northwestern town of Saraqeb, a monitor said, adding that 10 civilians were killed in southern Idlib province.
The Britain-based Syrian Observatory for Human Rights reported a "foul smell after government helicopters struck several areas of the town in Idlib province, causing five civilians to suffer from suffocation".
It quoted residents and medical sources as saying "toxic gas" was used in the attack, without elaborating.
Syrian government air strikes also killed six civilians in the town of Kafr Nabi, near Maaret al-Numan in the southern countryside of Idlib province, the Observatory said.
#Saraqeb@SyriaCivilDef teams respond to an attack with chlorine gas. 9 injured including 3 White Helmet volunteers. Attacks like this, in violation of UN Security Council resolutions, happen with impunity. @BBCWorld@cnnbrkpic.twitter.com/mLtfQ0OMnv
— The White Helmets (@SyriaCivilDef) February 4, 2018
Four other civilians were killed in government bombing on Maaret al-Numan and Maasarin.
The Observatory also said that “airplanes believed to be Russian hit the main hospital in Maaret al-Numan, causing damage”.
The facility shut down until repairs could be carried out, the war monitor said.
Russian jets intensified their raids on Sunday night, a day after rebels shot down a Russian warplane and killed its pilot.
The latest developments come as the United States recently accused the Syrian government of using chemical weapons on opposition forces near the capital Damascus.
The Syrian foreign ministry denied the accusations as "lies".
US Defence Secretary Jim Mattis told reporters on Friday his government was concerned sarin gas may have been recently used in Syria, citing reports from NGOs and rebel groups.
These reports said toxic gas has been used.
Mattis said the United States has no proof yet to support these accusations.
Last month, 21 people were treated for respiratory problems after rockets were fired on the rebel bastion of Eastern Ghouta outside Damascus.