Syrian President Bashar al-Assad says claims his government launched a chemical attack on his own people are "100 per cent fabrication".

In his first interview since the attack, Assad said he believed that the US was working "hand in glove with the terrorists" and maintained that Syria gave up all its chemical weapons in 2013.

"There was no order to make any attack. We gave up our arsenal a few years ago. Even if we have them, we wouldn't use them," he said. "Our impression is that the West, mainly the United States, is hand-in-glove with the terrorists. They fabricated the whole story to have a pretext for the attack."

The suspected attack killed at least 87 people, including many children, and images of the dead and of suffering victims provoked global outrage.

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Assad insisted it was "not clear whether it happened or not, because how can you verify a video? You have a lot of fake videos now."

Tests have confirmed sarin gas was used in Khan Sheikhoun, Syria. Photo / AP
Tests have confirmed sarin gas was used in Khan Sheikhoun, Syria. Photo / AP

"We don't know whether those dead children were killed in Khan Sheikhun. Were they dead at all?"

He said Khan Sheikhun had no strategic value and was not a battle front.

"This story is not convincing by any means."

He said a barrage of US missiles against a military airport had not diminished his government's ability to carry out strikes.

"Our firepower, our ability to attack the terrorists hasn't been affected by this strike," he said.

Meanwhile, the US has admitted that it killed 18 allied fighters in northern Syria in a misdirected air strike.

"The strike was requested by the partnered forces, who had identified the target location as an Isis fighting position," the Pentagon said. "The target location was actually a forward Syrian Democratic Forces fighting position."

Assad said he would only allow an "impartial" external investigation of last week's suspected chemical attack.

"We can only allow any investigation when it's impartial, when we make sure that unbiased countries will participate in this delegation to make sure that they won't use it for politicised purposes," he said.

Assad said peace talks on resolving his country's war were ineffective because Washington was "not serious" about ending the conflict.

"The United States is not serious in achieving any political solution. They want to use it as an umbrella for the terrorists."

Global chemical weapons investigators have gone to Turkey to collect samples as part of an inquiry into the attack in Syria that killed 87 people on April 4.

The fact-finding mission was sent by the Organisation for the Prohibition of Chemical Weapons (OPCW) in The Hague to gather biometric samples and interview survivors, sources told Reuters.

The toxic gas attack, which killed scores of people including children, prompted a US cruise missile strike on a Syrian air base and widened a rift between the US and Russia, a close ally of Assad's in his conflict with rebels and militants.

Syrian authorities have repeatedly denied using chemical weapons. Russian officials said the gas had been released by an air strike on a poison gas storage depot controlled by rebels.

Washington said that account was not credible and rebels have denied it. Samples taken from the poison gas site in Syria's Idlib governorate tested positive for the nerve agent sarin, the British delegation at the OPCW said on Thursday.

"UK scientists have analysed samples taken from Khan Sheikhoun. These have tested positive for the nerve agent sarin, or a sarin-like substance," the delegation said during a special session at the OPCW in The Hague.

The UK result confirmed earlier testing by Turkish authorities that concluded sarin had been used for the first time on a large scale in Syria's civil war since 2013.

The OPCW mission will determine whether chemical weapons were used, but is not mandated to assign blame.

Its findings, expected in three to four weeks, will be passed to a joint United Nations-OPCW chemical weapons investigation.

International investigators have concluded that sarin, chlorine and sulphur mustard gas have been used in Syria's 6-year-old conflict, with government forces using chlorine and Islamic State militants using sulphur mustard.