Accusations that chlorine gas has been used by Syrian government forces are to be investigated by chemical weapons inspectors.

Activists say canisters have been dropped in barrel bombs from helicopters – claims denied by Damascus.

The Organisation for the Prohibition of Chemical Weapons (OPCW) is to send a fact-finding mission, saying that President Bashar al-Assad’s government had agreed to accept the mission and had promised to provide security in areas under its control.

“The mission will carry out its work in the most challenging circumstances,” the OPCW said, referring to the three-year-old conflict between Assad’s forces and rebels. It gave no exact date for the mission but said it would take place soon.

Accusations by rebels and Syrian activist of at least three separate chlorine gas attacks by Assad’s forces in the last month have exposed the limits of a deal which Assad agreed last year for the destruction of his chemical arsenal.

The accord followed a sarin gas attack on rebel-held outskirts of Damascus last August in which hundreds of people were killed. Washington and its allies blamed Assad’s forces for the attack, but Damascus authorities said rebels carried it out to try to force Western military intervention.

Damascus has now shipped out or destroyed 92 percent of the chemicals it pledged to eliminate. However chlorine, which also has many industrial uses, was never included in the list submitted to the OPCW.

Videos released by activists of chlorine gas canisters they said were dropped in barrel bombs from Syrian military helicopters could not be verified by Reuters but analysts say the pattern of attacks suggest a coordinated campaign with growing evidence of government responsibility.

The US State Department said last week that if Syrian authorities used chlorine gas with the intent to kill or harm this would violate the Chemical Weapons Convention (CWC), which it joined as part of last year’s agreement.

Meanwhile the Syrian military have been accused of raining dozens of barrel bombs containing metal and explosives on Aleppo, causing massive casualties.

Unverified amateur pictures uploaded to the internet are all said to show the aftermath of an attack on Sunday.

They were purportedly filmed in several districts of the northern city.

President al-Assad’s forces have stepped up their bombardments in the region after making gains in Damascus and central Syria.

On Tuesday dozens of people were reported killed in a series of attacks on government-held areas in Homs and in the capital, where an education complex was hit.