Syrian President Bashar al-Assad said a suspected chemical weapons attack was a "fabrication" to justify a US military strike, AFP news agency reported.

In his first interview since the April 4 incident prompted a US cruise missile attack on Syrian forces, Assad insisted his army gave up all of its chemical weapons three years ago and that Syrian military power was not affected by the US strike.

"Definitely, 100 percent for us, it's fabrication," Assad said of the poison gas incident.

"Our impression is that the West, mainly the United States, is hand-in-glove with the terrorists. They fabricated the whole story in order to have a pretext for the attack," added Assad, who has been in power for 17 years.

Western leaders including US President Donald Trump have accused Assad of being behind last week's attack in the rebel-held town Khan Sheikhoun, saying his forces unleashed a chemical weapon during an air strike.

Assad said his forces had not been diminished by the 59 US cruise missiles launched at an Syrian airbase in Homs in retaliation for what Trump called a "very barbaric" attack.

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

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

At the time of the incident Syria denied any use of chemical weapons, and Moscow said the deaths had been the result of a conventional strike hitting a rebel arms depot containing "toxic substances".

But in his latest interview, Assad insisted it was "not clear" whether an attack on Khan Sheikhoun had even happened.

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"You have a lot of fake videos now," he said. "We don't know whether those dead children were killed in Khan Sheikhoun. Were they dead at all?"

An open source investigation by journalist Elliot Higgins pieces together the timeline of the Khan Sheikhoun attack and attempts to debunk previous claims by the government in Damascus and Russia that the chemical cloud was released after Syrian jets targeted a rebel weapons depot in the town.

US military officials said they observed a drone flying over a hospital in Khan Sheikoun that was providing treatment for gas victims.

"About five hours later, the UAV [drone] returned, and the hospital was struck by additional munitions," one official said.

The Organisation for the Prohibition of Chemical Weapons (OPCW) has begun an investigation into the alleged attack, but Russia on Wednesday blocked a UN Security Council resolution demanding Syria cooperate with the probe.

A British delegation at the world's chemical weapons watchdog said on Thursday that samples taken from the attack site tested positive for the nerve agent sarin.

Assad said Syria would only allow an "impartial" investigation into the poison gas incident.

He insisted several times his forces had turned over all chemical weapons stockpiles in 2013, under a deal brokered by Russia to avoid threatened US military action.

The OPCW has blamed Assad's government for at least two attacks in 2014 and 2015 involving the use of chlorine.

In the lead up to the government takeover of the rebel-held half of Aleppo last last year, Human Rights Watch reported on the government's "systematic" use of chlorine gas, marking at least eight incidents in which military helicopters dropped the chemical into residential areas.