Pentagon chief says Assad took Trump's chemical warning 'seriously'

US President Donald Trump's warning to the Syrian government not to carry out a chemical weapons attack appears to have worked, Pentagon chief Jim Mattis said Wednesday.

"It appears that they took the warning seriously," Mattis said, referring to the regime of Syrian President Bashar al-Assad.

White House spokesman Sean Spicer warned Monday night that if "Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."

The warning came after US intelligence noticed suspect activity at the airbase used to launch a suspected chemical strike two months ago.

"I think the president speaking about (these preparations) says how seriously we took them. He wanted to dissuade them," Mattis told reporters as he flew into Brussels for a NATO defence ministers meeting.

The April 4 attack on the rebel-held town of Khan Sheikhun was reported to have killed at least 87 people, including many children, and images of the dead and of suffering victims provoked global outrage.

Washington launched a retaliatory cruise missile strike days later against the Shayrat airbase -- the first direct US action against the regime, which denies any use of chemical weapons.

When asked how he knew Trump's warning had worked, Mattis said: "They didn't do it," a reference to the fact no chemical strike had occurred since Monday.

At the same time, he cautioned that "Assad's chemical programme goes far beyond one airfield."