Syrian troops have deployed bombs containing a killer gas while fighting rebels in the central city of Homs, the Syrian Observatory for Human Rights and activists said on Monday.
"Activists in Homs say that six rebels died on Sunday night on the Khaldiyeh-Bayada frontline because they inhaled odorless gas and white smoke," said the Britain-based Observatory, which relies on a network of activists, lawyers and doctors to document Syria's raging conflict.
"Gas spread in the area after regime troops threw bombs that gave off white smoke as soon as they hit the walls," said the Observatory, which added that the bombs were deployed during street clashes with the rebels.
"Those who inhaled the gas felt nauseous and suffered severe headaches. Some suffered fits," it added.
"These are not chemical weapons, but we do not know whether they are internationally prohibited," Observatory director Rami Abdel Rahman told AFP.
"Activists say it is the first time they have recorded these effects," he added. "They are not conventional weapons."
The Observatory called on the International Committee of the Red Cross to urgently send a specialised medical team to Homs, several of whose districts have been besieged by army forces for more than six months.
The Local Coordination Committees also reported the use of "bombs containing gases" in Homs.
"These gases lead to muscle relaxation, severe difficulty in breathing and the narrowing of the iris," said the LCC, a grassroots network of peaceful activists.
Amateur video filmed by activists and distributed online by the LCC showed a man laid out on a stretcher struggling to breathe as an unidentified doctor held an oxygen mask over his face.
"It's definitely a poisonous gas, but we don't know what type it is," said a field doctor.
"This man has been injured by the gas. We do not know what type of gas it is. It is definitely not sarin," he added.
US President Barack Obama led international warnings to President Bashar al-Assad over Syria's chemical weapons arsenal but Damascus has repeatedly insisted it would not use the arms against its own people.