World Bulletin / News Desk

United Nations investigators on Thursday blamed a sarin gas massacre on Bashar al-Assad's regime, as the United States renewed its warning that he has no role in Syria's future.

More than 80 people died on April 4 this year when sarin gas projectiles were fired into Khan Sheikhun, a rebel-held town in the Idlib province of northwestern Syria.

Images of dead and dying victims, including young children, in the aftermath of the attack provoked global outrage and a US cruise missile strike on a regime air base. 

The UN placed the death toll at 83 while the UK-based Syrian Observatory for Human Rights said it was 87.

Syria and its ally Russia had suggested that a rebel weapon may have detonated on the ground but the UN panel confirmed Western intelligence reports that blamed the regime.

"The panel is confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhun on 4 April 2017," the report, seen by AFP, says.

The report will increase pressure on Assad's regime just as Washington, in the wake of battlefield victories against the ISIL group, renews calls for him to step down.

Secretary of State Rex Tillerson's comments to reporters came during a visit to Geneva in which he met UN envoy Staffan de Mistura, who is trying to convene a new round of peace talks next month.

The secretary said US policy has not changed, but his remarks represented tougher language from an administration that had previously said Assad's fate is not a priority.

"We do not believe there is a future for the Assad regime, the Assad family," Tillerson said.

"I think I've said it on a number of occasions. The reign of the Assad family is coming to an end, and the only issue is how should that be brought about."

Russia, which is running a parallel peace process with Iran and Turkey in a series of talks in the Kazakh capital Astana, reacted coolly to Tillerson's remarks.

"I think we should not pre-empt any future for anybody," said Moscow's UN ambassador Vassily Nebenzia, who on Tuesday had vetoed a US attempt to extend the gas attack probe.

British Foreign Secretary Boris Johnson said the UN panel's report had reached a "clear conclusion" and urged the "international community to unite to hold Assad's regime accountable."

"I call on Russia to stop covering up for its abhorrent ally and keep its own commitment to ensure that chemical weapons are never used again," he said.