More than a dozen Syrians died possibly as a result of chemical weapons use in northern Syria on Tuesday in what, if confirmed, would be the first use of such arms in the country’s two year civil war.

The United Kingdom and the United States have previously warned that the use of chemical weapons on the part President Bashar al-Assad’s regime, which is battling a largely Sunni uprising, would cross a “red line,” although without stipulating what the consequences could be.

Syrian state television accused rebel forces of firing a rocket carrying chemical agents that killed 25 although there has been no suggestion that nongovernment forces were in possession of such weapons.

The Syrian Observatory for Human Rights, a group that sympathizes with the opposition, said sixteen soldiers were among the perished. A photographer employed by the Reuters news agency reported victims in Aleppo hospitals were suffering breathing problems and others had said they could smell chlorine after the alleged attack.

Russia, which has criticized Western powers’ condemnations of the Assad regime’s suppression of the uprising, was the only foreign country to confirm the use of chemical weapons as of Tuesday night and blamed the opposition for it. A spokeswoman for Britain’s Foreign Office said such weapons use would “demand a serious response from the international community and force us to revisit our approach so far.”

Britain and France have pushed their European Union allies in recent weeks to lift a weapons embargo that prevents them from arming opposition groups which they claim are outgunned by loyalist forces.