* OPCW say inspectors arrived in Syria
* Footage shows green-yellow gas in street
* Damascus denies using chemicals, blames rebels
* Photographer describes attack, says 70 wounded (Adds OPCW mission inside Syria, diplomat quote)
By Oliver Holmes
BEIRUT, May 23 (Reuters) - Opposition activists have posted a video of what they say is chlorine gas floating through the streets of a Syrian village and they accuse President Bashar al-Assad of mounting a chemical weapons campaign.
Activists and medical sources said the village of Kfar Zeita, in the central province of Hama 125 miles (200 km) north of Damascus, has been the focus of a two-month-old assault in which chlorine gas canisters have been dropped out of helicopters.
The government denies its forces have used chlorine or deadlier poisonous gases and blames all chemical attacks on rebels waging a three-year-old uprising against Assad.
The Organisation for the Prevention of Chemical Weapons (OPCW) has opened an investigation into the alleged chlorine attacks, more than a dozen of which have been reported since April 11 in several areas.
"Inspectors for the fact-finding mission arrived in Syria last week and are trying to get to some of the sites," the OPCW told Reuters.
Text accompanying the opposition video, posted by a user called Mustapha Jamaa, said it was filmed on Thursday in Kfar Zeita by the Revolution General Commission 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."
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 freelance photographer told Reuters 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."
Activists said Kfar Zeita was attacked twice on Thursday, as well as the village of Al-Tamana'a in northwest Idlib province.
The alleged attack took place on the same day that Russia and China vetoed a resolution to refer the situation in Syria to the International Criminal Court for possible prosecution of war crimes and crimes against humanity.
Assad agreed with the United States and Russia to dispose of his chemical weapons after hundreds of people were killed in a sarin gas attack on the outskirts of the capital last August.
Chlorine is likely to be lethal than sarin but its use as a weapon is still illegal under a global chemical weapons convention that Syria signed.
Its use would also breach the terms of the deal with Washington and Moscow, itself now weeks behind schedule, with roughly seven percent of Assad's chemical arsenal still inside Syria.
A diplomat in the region said that 15 OPCW inspectors were stuck in Damascus while they negotiate access with the government and rebels.
"They are negotiating security guarantees from all sides. That's a challenge with opposition groups," he said.
Syria did not declare chlorine as part of its stockpile, further complicating the operation to rid Assad of chemical weapons.
Syria's civil war started with a pro-democracy movement against Assad that armed itself following attacks by security forces on protesters.
The ensuing conflict has killed more than 160,000 people, a third of them civilians, and caused millions to flee.
(See footage here: https://www.youtube.com/channel/UCMs7nZ3OfQbTgOC_iqxufkg) (Additional reporting by Anthony Deutsch in The Hague and Dominic Evans in Beirut; Editing by Angus MacSwan)