A new report by the UN and the chemical watchdog says the Syrian government was responsible for a chemical attack that killed scores of people in April.

More than 80 people, including many women and children, died when, on April 4, the nerve agent sarin or a sarin-like substance was dropped onto the opposition-held town of Khan Sheikhoun in the northern province of Idlib.

The United States launched a retaliatory cruise missile strike days later against a Syrian airbase from where it alleged the chemical weapons attack was launched.

The report from the United Nations and Organisation for the Prohibition of Chemical Weapons's (OPCW) Joint Investigative Mechanism (JIM) is expected to be released later on Thursday.

"We are told by at least two diplomats here at the UN that the report by the JIM does hold the Syrian government responsible for that sarin gas attack," Al Jazeera's Mike Hanna, reporting from the UN headquarters in New York, said.

"That was the position adopted by, among others, the US, the UK and France in the wake of that attack," he added.

"They were absolutely adamant that the Syrian government was responsible at the time - hence the attack that was carried out a few days later by the US."

Hanna said it was important to notice that the fact-finding mission, JIM's investigative arm, did not actually go to the site of the attack.

"They said that the security situation was too dangerous to actually go there, however, they contend that they gathered enough concrete evidence to be able to make this conclusion," he added.

Syria denials

There was no immediate reaction from the Syrian government of President Bashar al-Assad.

Syria has previously denied involvement, claiming it no longer possesses chemical weapons following a 2013 deal under which it pledged to surrender its chemical arsenal.

But last month, the UN Commission of Inquiry (COI) on Syria said it had gathered an "extensive body of information" showing that the Syrian air force was responsible for the attack on Khan Sheikhoun.

The investigators said they had based their findings on photographs of bomb remnants, satellite imagery and witness testimony.

That COI report came after a fact-finding mission by the OPCW concluded that sarin or a sarin-like substance was used in the attack, but did not assign blame.

Russia, which in September 2015 launched an air campaign in Syria in support of Assad's force, has also insisted in the past that there is not sufficient evidence to prove who was behind it.

Moscow "pointed out that members of the fact-finding mission didn't go to the area itself", said Hanna.

"However, within the report that will be released, the JIM makes it very clear that there is enough evidence gathered despite the absence of inspectors from the site itself," our correspondent added.