Syria’s government and rebels accused each other of launching a deadly chemical attack near the northern city of Aleppo on Tuesday in what would, if confirmed, be the first use of such weapons in the two-year-old conflict.

US President Barack Obama has warned Assad in the past that any use of chemical weapons would be a “red line”. There has however been no suggestion of rebels possessing such arms. 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.

The pro-opposition Syrian Observatory for Human Rights, which monitors the conflict, put the number of dead at 26, including 16 soldiers. The reported death toll is far below the mass slaughter inflicted on the Iraqi Kurdish city of Halabja where an estimated 5,000 people died in a chemical attack ordered by  Saddam Hussein 25 years ago.