Human Rights Watch on Friday urged the international community to slap sanctions on the Syrian government after UN investigators blamed President Bashar al-Assad's regime for a sarin gas attack that killed dozens.

"The (UN) Security Council should move swiftly to ensure accountability by imposing sanctions on individuals and entities responsible for chemical attacks in Syria," the New York-based rights watchdog said in a statement.

The April 4 attack in which sarin gas projectiles were fired into Khan Sheikhun, a rebel-held town in Idlib province in northwestern Syria, killed 83 people, according to the United Nations.

The Syrian Observatory for Human Rights gave a death toll of 87, including more than 30 children.

A UN panel of investigators said Thursday in a report it was "confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhun", an attack which prompted a retaliatory US strike on a Syrian air base.

Ole Solvang, deputy emergencies director at HRW, said the panel's report "should end the deception and false theories that have been spread by the Syrian government".

"Syria's repeated use of chemical weapons poses a serious threat to the international ban against the use of chemical weapons," Solvang said.

"All countries have an interest in sending a strong signal that these atrocities will not be tolerated."

UN experts have also accused the Syrian regime, in a war with rebel forces for the past six years that has cost more than 330,000 lives, of launching chlorine gas attacks in the north of the country in 2014 and 2015.