France has said it has proof Bashar al-Assad's government was behind the recent sarin chemical weapons attack in Syria.

Foreign affairs minister Jean-Marc Ayrault said France had made the determination based on comparing samples from another attack in 2013.

He said: "We know, from a certain source, that the process of fabrication of the samples taken is typical of the method developed in Syrian laboratories.

"This method is the signature of the regime and it is what enables us to establish the responsibility of the attack. We know because we kept samples from previous attacks that we were able to use for comparison."

The attack on the village of Khan Sheikhoun, on 4 April, killed scores of people including children.

It has been claimed the regime still has a stockpile hundreds of tonnes of chemical weapons despite saying it had handed over all of them.

French intelligence agents are aware that a Syrian Sukhoi 22 warplane struck Khan Sheikhoun six times on the day of the attack, Mr Ayrault said.

There is no doubt that sarin used there was manufactured in Syrian laboratories, he added.