A United Nations (U.N.) chemical weapons expert, wearing a gas mask, holds a plastic bag containing samples from one of the sites of an alleged chemical weapons attack in the Ain Tarma neighbourhood of Damascus. (Reuters Photo)

DAMASCUS, Sept. 6 (Xinhua) -- A UN probe held Syrian government forces responsible for a chemical attack which killed dozens of people in April, the Syrian Observatory for Human Rights said Wednesday.

The UN war crimes investigators said Wednesday that they had evidence Syrian government forces were behind a chemical attack which killed dozens of people in Khan Sheikhun in April, the Syrian Observatory for Human Rights said.

The UN Commission of Inquiry (COI) on Syria said it had gathered an "extensive body of information" showing that the Syrian airforce was behind the horrific sarin gas attack on April 4.

"All evidence available leads the commission to conclude that there are reasonable grounds to believe Syrian forces dropped an aerial bomb dispersing sarin in Khan Sheikhun," the report said.

At least 83 people, a third of them children, were killed and nearly 300 wounded in the attack on Khan Sheikhun, a town in the opposition-held northern province of Idlib, it said.

The Syrian Observatory for Human Rights, a Britain-based monitor, has previously given a death toll of at least 87.

Syria's government has denied involvement and claims it no longer possesses chemical weapons after a 2013 agreement under which it pledged to surrender its chemical arsenal.