Britain on Friday urged the United Nations Security Council to impose sanctions on Syria after a UN-led investigation found that government forces were responsible for the sarin gas attack on Khan Sheikhun.

A joint UN-Organisation for the Prohibition of Chemical Weapons (OPCW) panel said the Syrian air force had dropped a bomb on the opposition-held town on April 4, releasing the deadly nerve agent that killed more than 80 people.

"A robust international response is now essential to hold those responsible for the Khan Sheikhun attack to account," British Ambassador Matthew Rycroft told reporters.