Syrian opposition groups say possibly hundreds of people were killed today in chemical weapons attacks on rebel-held suburbs of Damascus.

Videos uploaded to YouTube by activists show people, including many children, being treated in makeshift hospitals, reports the BBC, which has been unable to fully verify the videos but believes they are genuine. The videos show some victims having convulsions, while others appear to be having difficulty breathing.

The country's state-run news agency calls reports of the use of toxic gas "baseless" and accuses the rebels of trying to distract United Nations weapons inspectors who arrived in the country on the weekend.

"We thought this regime would not use chemical weapons, at least these days with the presence of the UN inspectors," an activist who witnessed the aftermath of the attack and believes the death toll is in the hundreds tells the New York Times. "It is reckless. The regime is saying, 'I don’t care.'"

The UK and the Arab League have urged the regime to allow the inspectors to visit the site of the government offensive.