The French newspaper Le Monde says its reporters persuaded Syrian doctors to hand over samples of blood, urine and hair from sickened rebel fighters by promising to analyze them for toxin.

The journalists turned the samples over to the French government, which on Tuesday said that traces of the nerve agent sarin in them were the first tangible proof that Syria's government was using chemical weapons.

According to Wednesday's issue of Le Monde, the Syrian doctors had been carefully collecting the samples from sick rebels for some time, hoping to spirit them abroad for analysis. It took Le Monde about a month to get them out of Syria, the paper said.