There is "absolutely no doubt" the government of Bashar al-Assad was behind the use of chemical weapons in Syria in April, Britain's foreign minister Boris Johnson said on Friday.

Axar.az reports citing Reuters.

The world's chemical weapons watchdog said the banned nerve agent sarin was used in an attack in northern Syria that killed dozens of people, a report from a fact-finding team seen by Reuters showed.

Western intelligence agencies had blamed Assad's government for the attack, but Syrian officials have repeatedly denied using banned toxins in the conflict.

"The exact responsibility for dropping the sarin will now go to a joint investigative mechanism to be confirmed, but I've got absolutely no doubt that the finger points at the Assad regime," Johnson said on Sky News.

"We will drive on with the UK campaign to impose sanctions on those responsible... People who drop chemical weapons on innocent people should be held to account."