US President Donald Trump told allies it was time to end Syria's "brutal" civil war Wednesday, as he branded the country's leader Bashar al-Assad a "butcher" and questioned Russia's role in a suspected chemical attack.

Trump, standing alongside NATO Secretary General Jens Stoltenberg, called on allies to "work together to resolve the disaster" in Syria and thanked them for condemning Assad's suspected sarin attack in Khan Sheikhun.

"Vicious slaughter of innocent civilians with chemical weapons including the barbaric killing of small and helpless children and babies must be forcefully rejected by any nation that values human life," Trump told reporters.

"That's a butcher. That's a butcher. So I felt we had to do something about it. I have absolutely no doubt we did the right thing, and it was very, very successfully done," he added.

"It is time to end this brutal civil war, defeat terrorists and allow refugees to return home."

Trump's comments came shortly after Russia vetoed a UN Security Council resolution that would have compelled Damascus to cooperate with an investigation of the attack.

Trump said it was "certainly possible" that Russia President Vladimir Putin knew about the attack, blamed on Assad, indicating Russian officials were present at the source airbase, which Trump later bombed.

"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.

Trump also praised China for abstaining during the UN vote. He met President Xi Jinping last week in Florida and spoke again to the Chinese leader on Tuesday.

"I think it's wonderful that they abstained," he said. "We're honored by the vote. That's the vote that should have taken place."

Source: AFP