Syrian opposition activists have posted a video of what they say is chlorine gas floating through the streets of a village, the first such footage of what they say is a chemical weapon campaign by President Bashar al-Assad.

The village of Kfar Zeita, in the central province of Hama 200 kilometres north of Damascus, has been the epicentre of what activists and medics call a two-month-old assault in which chlorine gas canisters have been dropped out of helicopters.

Damascus denies that forces loyal to Assad have used chlorine or other more poisonous gases and blames all chemical attacks on rebels fighting them in a three-year-old uprising. Text accompanying the video, posted by a user called Mustapha Jamaa, said it was filmed on Thursday in Kfar Zeita by the Revolution General Commission, an opposition group.

It showed green-yellow gas in a street. A man runs away from the gas cloud with a woman who is holding a cloth to her mouth. Another man in camouflage trousers and wearing a gas mask calls out for a car to assist the woman. A voice off screen says: “Chlorine gas bombing. Yellow smoke.”

The smell of chlorine was very obvious

Reuters could not independently verify the authenticity of the footage. The video did not show an impact site or indicate where it was filmed.

A Reuters freelance photo­grapher said he arrived at the scene of the attack an hour after a helicopter dropped the bomb.

“The smell of chlorine was very obvious. It smelt like vinegar, or bleach. I started to cough and hyperventilate. My eyes were burning,” he said.

One of his photos showed the woman who was running away from the gas in the video. She was being treated with oxygen at a field hospital. “There were 70 wounded people,” he said. “Those who were at the impact site fainted.”

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