Syria's government and rebels have accused each other of launching a deadly chemical attack near the northern city of Aleppo.
If confirmed, it would be the first use of such weapons in the two-year-old conflict.
Syria's information minister said rebels had fired a rocket carrying chemical agents that killed 16 people and wounded 86.
State television said later the death toll had risen to 25.
A senior rebel commander, Qassim Saadeddine, blamed President Bashar al-Assad's forces for the alleged chemical strike.
A Reuters photographer said victims he had visited in Aleppo hospitals were suffering breathing problems and that people had said they could smell chlorine after the attack.
"I saw mostly women and children," said the photographer, who cannot be named for his own safety.
"They said that people were suffocating in the streets and the air smelt strongly of chlorine."
The photographer quoted victims he met at the University of Aleppo hospital and the al-Rajaa hospital as saying: "People were dying in the streets and in their houses."
Mr Assad, who is battling an uprising against his rule, is widely believed to have a chemical arsenal.
Syrian officials have neither confirmed nor denied this, but have said that if it existed it would be used to defend against foreign aggression, not against Syrians.
There have been no previous reports of chemical weapons in the hands of insurgents.
US President Barack Obama, who has resisted overt military intervention in Syria's civil war, has warned Mr Assad that any use of chemical weapons would be a "red line".
The US has also expressed concern about chemical weapons falling into the hands of militant groups - either hardline Islamist rebels fighting to topple Mr Assad or his regional allies.