French analysis finds Assad regime was behind sarin gas attack in Syria

Russia, a close ally of Assad, 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 evidence of a poison gas attack is made up.

But Mr 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 added, saying that France is working to bring those behind the "criminal" atrocities to international justice.

France's Foreign Ministry said blood samples were taken from a victim in Syria on the day of the April 4 attack in the opposition-held town of Khan Sheikhoun, which killed more than 80 people.

Environmental samples, the ministry said, show the weapons were made "according to the same production process as the one used in the sarin attack perpetrated by the Syrian regime in Saraqeb" on April 29, 2013.

Mr Ayrault said French intelligence showed that only Syrian government forces could have launched such an attack - by a bomber taking off from the Shayrat air base, which was later targeted in a retaliatory US missile strike.

France's presidency said the country's intelligence services presented evidence showing the Syrian government "still holds chemical warfare agents, in violation of the commitments to eliminate them that it took in 2013". It said that information will be made public, without offering details.

It is thought that Assad's government still has a stockpile of tons of chemical weapons, despite saying it had handed over all of them.

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

Russia has previously called for an international probe, and Mr Peskov expressed regret that the Organisation for the Prohibition of Chemical Weapons (OPCW) has turned down the Syrian government's offers to visit the site of the attack and investigate.

The French minister's comments came as the OPCW, which is investigating the April 4 attack, held a ceremony in The Hague marking the 20th anniversary of the Chemical Weapons Convention.

In a video message to the ceremony, UN Secretary-General Antonio Guterres said the organisation's progress over two decades seeking to eliminate chemical weapons is now under threat.

"In the Middle East, belligerents are breaking the norm against chemical weapons," he said. "The recent attack in Syria was a horrific reminder of the stakes. There can be no impunity for these crimes."

Meanwhile, a series of Turkish air strikes on Tuesday killed 20 Syrian Kurdish fighters in attacks that Ankara said targeted Kurdish rebel positions in Syria and Iraq.

Syrian Kurdish officials escorted an American officer to some of the sites targeted in the attack, as large crowds from the area followed them around, according to photos and video from the scene. Redur Khalil, the spokesman for the Syrian Kurdish People's Protection Units, or YPG, confirmed the visit.

Mr Khalil said the air strikes were followed on Wednesday by Turkish army shelling of Syrian villages along the border area. The shelling prompted an exchange of fire between Kurdish and Turkish border posts, Mr Khalil said.

The YPG form the backbone of the Syrian Democratic Forces (SDF), the main US partner in the battle against the Islamic State group in northern Syria. Nato member Turkey considers the YPG an extension of an insurgency within its own borders. The SDF includes Arab fighters.

Mr Khalil said the Turkish escalation would "obstruct the war" against IS.

The Syrian Foreign Ministry condemned the air strikes. The SDF and the Syrian government have largely avoided confrontations with each other over the course of the civil war. No Syrian government forces were targeted in the attack.

AP