UN war crimes investigators say they have evidence that Syrian government forces were behind a chemical attack that killed scores of people in a rebel-held town in April.

The UN Commission of Inquiry (COI) on Syria said on Wednesday it had gathered an "extensive body of information" showing that the Syrian air force was responsible for the sarin gas attack in Khan Sheikhoun.

"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 Sheikhoun," the report said.

At least 83 people, a third of them children, were killed and nearly 300 wounded in the attack on the town in the northern province of Idlib, it said. Other sources have given a death toll of at least 87.

READ MORE: Syria's civil war explained from the beginning

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.

A fact-finding mission by the UN's chemical watchdog, the OPCW, concluded earlier this year that sarin gas was used in the attack, but did not assign blame.

A joint UN-OPCW panel is currently working to determine whether Syrian government forces were behind the attack.

Significant evidence

But Wednesday's report is the first from the United Nations to officially lay blame for the attack on Damascus.

The report also found that the Syrian government was responsible for at least 23 other chemical attacks in the war-ravaged country since March 2013.

INTERACTIVE: From chlorine to sarin - Chemical weapons in war

The investigators, who have never been granted access to Syria, said they had based their findings on photographs of bomb remnants, satellite imagery and witness testimony.

They determined that a Su-22 fighter bomber, which is only operated by the Syrian air force, conducted four air attacks in Khan Sheikhoun at around 6:45am on April 4.

"The Commission identified three of the bombs as likely OFAB-100-120 and one as a chemical bomb," the report said, adding that "photographs of weapon remnants depict a chemical aerial bomb of a type manufactured in the former Soviet Union".

The investigators said they had found no evidence supporting Syrian and Russian claims that the chemicals had been released when an air raid hit an opposition weapons depot in the area producing chemical munitions.

Their report, which covers the period from March 1 to July 7, also found that Syrian government forces had carried out chemical attacks on at least three other occasions since March - in Idlib, Hamah and eastern Ghouta - using weaponised chlorine.

The report is the 14th from the COI, which has been tasked with detailing atrocities in the Syrian conflict that has killed hundreds of thousands of people since 2011.