The world's chemical arms watchdog said today it had found no evidence of nerve gas after an attack on the Syrian town of Douma, but chlorine may have been used.

A team of inspectors from the Organisation for the Prohibition of Chemical Weapons took more than 100 samples from some seven sites in Douma when they gained access to the town several weeks after the 7 April attack.

"The results show that no organophosphorous nerve agents or their degradation products were detected," the OPCW said in an interim report this evening.

But it added that the fact-finding mission also found "along with explosive residues, various chlorinated organic chemicals".

It is understood that could mean some samples contained what may be potential markers of exposure to an active source of chlorine, not found naturally in the environment.

"Work by the team to establish the significance of these results is ongoing," the OPCW added.

Medics and rescuers say about 40 people were killed in the attack, many of them in a housing block hit when a cylinder landed on its roof.

The fact-finding team was still working on "the location of the cylinder, (and) its provenance" which will require a "comprehensive analysis" by experts, the OPCW said.

The team's mission to Douma was launched amid international outrage over images of adults and children appearing to be suffering from the effects of a toxic weapon attack.

The gruesome footage from the apparent attack horrified the world and prompted unprecedented Western strikes on Syrian military installations. 

The fact-finding mission gained access to Douma on 21 April after several delays after deploying to Syria on 14 April.

There had been claims that residents were victims of sarin gas - but that has been ruled out by today's interim report.

But the Douma incident has deeply divided international opinion.

Russia has stuck by its ally Syria and insisted the attack was staged by the White Helmets volunteer rescue service.

Western powers, however, blamed the regime of Syrian President Bashar al-Assad. In response, the United States, France and Britain conducted missile strikes on Syrian military installations.