A man breathes through an oxygen mask as another one receives treatments, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah

PARIS (Reuters) - Samples obtained by French intelligence show that forces loyal to Syrian President Bashar al-Assad “undoubtedly” used sarin nerve gas in an April 4 attack in northern Syria, France’s foreign minister said on Wednesday, citing a declassified report.

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.

“We know, from a certain source, that the process of fabrication of the samples taken is typical of the method developed in Syrian laboratories,” Jean-Marc Ayrault told reporters.

“This method is the signature of the regime and it is what enables us to establish the responsibility of the attack. We know because we kept samples from previous attacks that we were able to use for comparison.”