* Sixth reported gas attack in Kfar Zeita village
* Damascus denies using chemicals, blames rebels
* U.S. Secretary of State Kerry has warned of consequences
BEIRUT, May 20 (Reuters) - Syrian opposition activists said on Tuesday forces loyal to President Bashar al-Assad had dropped a chlorine bomb on a rebel-held village, killing a teenager, the sixth alleged poison gas attack there in two months.
The village of Kfar Zeita, in the central province of Hama 125 miles (200 km) north of Damascus, has been the epicentre of what activists and medics say is a chemical weapons campaign in which chlorine gas canisters are 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 rebel forces fighting them in the three-year-old uprising.
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.
Activists from Kfar Zeita said on Facebook that Abdullah Qadour al-Hamawi, a disabled 14-year-old, was killed in the latest attack on Monday.
They posted video footage of men and children being treated in a field hospital. Many were lying down and one man appeared unresponsive. Reuters could not independently verify the authenticity of the footage.
Chlorine is thousands of times less lethal than sarin but is still illegal under a chemical weapons convention that Syria signed and its use would breach the terms of the deal with Washington and Moscow, itself now weeks behind schedule with 7.5 percent of Assad's chemical arsenal still inside Syria.
Syria did not declare chlorine as part of its stockpile, further complicating the operation to rid Assad of chemical weapons.
The watchdog Organisation for the Prevention of Chemical Weapons (OPCW) is considering launching its own fact-finding mission to investigate the reports of chlorine gas attacks, sources told Reuters last month.
U.S. Secretary of State John Kerry said on Thursday he had seen unverified raw data suggesting Syrian government forces had used chlorine gas.
"It has been made clear by President (Barack) Obama and others that use would result in consequences," Kerry said, adding: "We're not going to pin ourselves down to a precise time, date, manner of action, but there will be consequences."
Eliot Higgins, a British-based researcher who trawls daily through online videos of Syria's civil war to verify weapons in them, said at least five chlorine attacks had been alleged in Kfar Zeita and five in other rebel-held areas of the country.
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. (Reporting by Oliver Holmes; Editing by Andrew Heavens)