A report by French intelligence has concluded that forces loyal to Syrian President Bashar-al-Assad carried out the sarin nerve gas attack that sparked international condemnation.

The report outlined that the attack on the 4 April, was ordered either by Assad or by someone close to him.

The attack in the town of Khan Sheikhoun killed scores of people and prompted the United States to launch a cruise missile strike on a Syrian air base in response, its first direct assault on the Assad government in the conflict.

Among the elements found in the samples were hexamine, a hallmark of sarin produced by the Syrian government.

"The French intelligence services consider that only Bashar al-Assad and some of his most influential entourage can give the order to use chemical weapons," the report said.

Assad's claim to AFP news agency on April 13 that the attack was fabricated, was "not credible" given the mass flows of casualties in a short space of time arriving in Syrian and Turkish hospitals as well as the sheer quantity of online activity showing people with neurotoxic symptoms, said the report.

It was also noted in the report that jihadist groups in the region did not have the capability to develop or launch such an attack – although Islamic State is not known to be in the area.

The report was able to reach its conclusion based on samples they had obtained from the impact strike on the ground, and a blood sample from a victim.