US President Barack Obama, who has resisted overt military intervention, has warned Syrian President Bashar al 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 tele-vision said later the death toll had risen to 25.

The pro-opposition Syrian Observatory for Human Rights, which monitors the conflict using a network of contacts, 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 5000 people died in a chemical attack ordered by former Iraqi President Saddam Hussein 25 years ago.

There was no immediate confirmation from Western governments or inter-national organisations of a chemical attack, but Russia, an ally of Damascus, accused rebels of carrying out such a strike.

"We are very seriously concerned by the fact weapons of mass destruction are falling into the hands of the rebels, which further worsens the situation in Syria and elevates the confrontation in the country to a new level," the Russian Foreign Ministry said in a statement.

Britain said its calculations would change if an attack had taken place. "The UK is clear that the use or proliferation of chemical weapons would demand a serious response from the international community and force us to revisit our approach so far," the Foreign Office said.