Syrian President Bashar al-Assad said a suspected chemical weapons attack was a "fabrication" to justify a US military strike, as Moscow digs in to defend its ally despite increasing strains with Washington.

In an exclusive interview with AFP in Damascus — his first since the alleged April 4 attack prompted a US air strike on Syrian forces — Assad said his army had given up all its chemical weapons and that Syrian military power was not affected by the US strike.

"Definitely, 100 percent for us, it's fabrication," he said in the interview on Wednesday in reference to the alleged chemical weapons attack.

"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."

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

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

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."

'Fake videos'

In the interview, Assad insisted it was "not clear" whether an attack on Khan Sheikhun had even happened.

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

Related: Syria's Bashar al-Assad: Rex Tillerson is 'hallucinating'

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

"There was no order to make any attack, we don't have any chemical weapons, we gave up our arsenal a few years ago," Assad said.

He said his forces had not been diminished by the US strike.

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

Denouncing a "very barbaric" attack, Trump ordered a strike that saw 59 Tomahawk cruise missiles slam into the airbase in central Syria from where Washington accused Assad's forces of launching the attack.

Trump bats aside criticism

It was the first direct US military action against Assad's forces since the start of Syria's civil war six years ago and led to a quick downward spiral in ties between Washington and Moscow.

Russia accused the United States of breaking international law with the strike against the Syrian regime, a key ally that Moscow has supported with air strikes since 2015.

Trump gave such criticism short shrift on Wednesday, saying: "I felt we had to do something about it. I have absolutely no doubt we did the right thing."

Standing alongside NATO Secretary General Jens Stoltenberg in Washington, Trump also said it was "certainly possible" that Russia was aware of the suspected attack.

"I would like to think that they didn't know, but certainly they could have. They were there. So we'll find out," he said.

The strains in ties were clear as US Secretary of State Rex Tillerson received a frosty reception Wednesday on a visit to Moscow.

"There is a low level of trust between our two countries. The world's two foremost nuclear powers cannot have this kind of relationship," Tillerson told a Moscow news conference.

Russian Foreign Minister Sergei Lavrov agreed there were many problems, but stressed that Moscow was "open to dialogue with the US in all different areas."

Russia vetoes resolution

The polarised positions were evident too at the UN Security Council on Wednesday, when Russia vetoed a Western-drafted resolution that would have required Syrian cooperation in an investigation into the suspected chemical attack.

It was the eighth time that Russia has used its veto power to block action directed at Damascus. China opted to abstain, a move Trump praised.

British Foreign Secretary Boris Johnson said the veto "puts Russia on the wrong side of the argument," while French President Francois Hollande warned Russia it "bears a heavy responsibility" for continuing to protect Assad.

In a show of continued support for the regime, Moscow will host Syrian Foreign Minister Walid Muallem for talks with Lavrov on Thursday.

On Friday the two will join a three-way meeting with Foreign Minister Mohammad Javad Zarif of Iran, another key ally of the Assad regime.

Since breaking out with mass protests in March 2011, Syria's conflict has spiralled into a devastating civil war that has left more than 320,000 people dead and forced millions from their homes.

The war has drawn in a wide of range of global powers, from Russia and Iran in support of the regime, to Western nations, Turkey and Arab Gulf states in support of various rebel forces.

The war also led to the emergence of the Islamic State jihadist group, which seized control of large parts of Syria and Iraq in 2014 but has since lost much of the territory it once controlled.

By AFP's Sammy Ketz and Rana Moussaoui in Damascus.