France claims it has scientific proof that the Syrian government was responsible for a suspected chemical attack that killed 87 people earlier this month.

Foreign Minister Jean-March Ayrault said on Wednesday that chemical analysis of samples taken from the rebel-held village of Khan Sheikhoun "bears the signature" of President Bashar al-Assad's government.

"There is no doubt about the responsibility of the Syrian regime given the way that the sarin used was produced," Ayrault told journalists after presenting a report on findings at a meeting of French defence chiefs.

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He said the substance France believes was used in the attack contains hexamine, a component that was also found in a gas attack in northwest Syria in 2013.

"We are now in a position to confirm that the sarin used on April 4 is the same sarin that was used in an attack in Saraqeb on April 29, 2013," he said.

Russia promptly denounced the French report, saying the samples and the fact the nerve agent was used are not enough to prove who was behind it.

Assad has repeatedly denied that his forces used chemical weapons and claimed that evidence of a poison gas attack was made up.

But Ayrault said France knows "from sure sources" that "the manufacturing process of the sarin that was sampled is typical of the method developed in Syrian laboratories".

"This method bears the signature of the regime and that is what allows us to establish its responsibility in this attack," he said, adding that France is working to bring those behind the "criminal" atrocities to international justice.

Ayrault also said that French intelligence services showed that only Syrian government forces could have launched such an attack by a bomber taking off from the Sharyat airbase.

"The regime's air force ... is the only one with these aerial capabilities," Ayrault said.

Speaking to reporters in Moscow, Kremlin spokesman Dmitry Peskov said Russia's position on the attack is "unchanged" and that "that the only way to establish the truth about what happened near Idlib is an impartial international investigation".

The United States has also blamed Assad's government for the April 4 attack in Khan Sheikhoun.

President Donald Trump's administration fired cruise missiles at a Syrian airbase in retaliation for the attack and issued sanctions on 271 people linked to the Syrian agency said to be responsible for producing non-conventional weapons.

Syria has strongly denied the accusations.