Syria's government and rebels accused each other of launching a deadly chemical attack near the northern city of Aleppo today in what would, if confirmed, be the first use of such weapons in the two-year-old conflict.
US president Barack Obama, who has resisted overt military intervention in Syria, has warned Dr 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 using a network of contacts in Syria, 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 former Iraqi president Saddam Hussein 25 years ago.
There was no immediate confirmation from Western governments or international 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 that 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 a chemical 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," a Foreign Office spokeswoman said.
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. He quoted victims at the University of Aleppo hospital and the al-Rajaa hospital as saying people were dying in the streets and in their houses.
President Bashar al-Assad, 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.
Information Minister Omran al-Zoabi said rebels fired "a rocket containing poison gases" at the town of Khan al-Assal, southwest of Aleppo, from the city's southeastern district of Nairab, part of which is rebel-held.
"The substance in the rocket causes unconsciousness, then convulsions, then death," the minister said.
But a senior rebel commander, Qassim Saadeddine, who is also a spokesman for the Higher Military Council in Aleppo, denied this, blaming Assad's forces for the alleged chemical strike.
"We were hearing reports from early this morning about a regime attack on Khan al-Assal, and we believe they fired a Scud with chemical agents," he told Reuters by telephone from Aleppo.
Washington has expressed concern about chemical weapons falling into the hands of militant groups - either hardline Islamist rebels fighting to topple Assad or his regional allies.
Reuters