Residents in the Syrian town of Saraqeb, Idlib province were hit by a suspected chlorine gas attack overnight.
Rescue workers in the area told The Associated Press that civilians and three of their volunteers have been injured, suffering suffocation and choking.
Local media are reporting up to 20 people dead, but this has yet to be confirmed by authorities.
The suspected attack occurred one day after an al-Qaeda offshoot claimed responsibility for the downing of a Russian fighter jet.
Syrian rebels shot down the Russian warplane on Saturday and killed its pilot on the ground after he ejected from the plane, Russia's defence ministry and Syrian rebels said.
The SU-25 came down in an area of northern Idlib province that has seen heavy air strikes and fighting on the ground between Syria's government forces backed by Russia and Iran, and rebel groups opposed to President Bashar al-Assad.
Syrians opposed to Assad see Russia as an invading force they blame for the deaths of thousands of civilians since Moscow joined the war on the side of the government in 2015.
The U.S. State Department said it had seen reports about the incident and allegations that the United States provided missiles to groups in Syria.
"The United States has never provided MANPAD missiles to any group in Syria, and we are deeply concerned that such weapons are being used," said State Department spokeswoman Heather Nauert.
"The solution to the violence is a return to the Geneva process as soon as possible and we call on Russia to live up to its commitments in that regards."
The Russian plane was shot down over the town of Khan al-Subl near the city of Saraqeb, close to a major highway where the Syrian army and Iranian-backed militias are trying to advance, a rebel source said.
Although the Russian pilot escaped the crash, he was killed by rebels who had tried to capture him, the source said.