PALM BEACH/WASHINGTON (REUTERS/WASHINGTON POST) - US Secretary of State Rex Tillerson said on Thursday (April 6) that the US government had no doubt that Syrian President Bashar al-Assad was responsible for a chemical weapons attack that killed scores of civilians, adding that there was no role for the latter in Syria in future.

"We are considering an appropriate response for this chemical weapons attack," Tillerson said in Palm Beach, Florida, where President Donald Trump will meet with Chinese President Xi Jinping.

"It is a serious matter. It requires a serious response," Tillerson said.

Asked about the future of the Syrian strongman, who has withstood a multi-faceted war now in its seventh year, Tillerson appeared to change tack from earlier Trump administration statements about Assad.

"Assad's role in the future is uncertain, clearly," he told reporters. "With the action he's taken, it would seem there would be no role for him to govern the Syrian people."

When asked whether the Trump administration would lead a coalition to remove Assad, Tillerson replied: "Those steps are underway."

It was not immediately clear what he meant, either about a coalition or about removing a foreign leader. He suggested, however, that he was speaking about a long process and not some sudden military move.

"The process by which Assad would leave is something that I think requires an international community effort to both first defeat ISIS within Syria to stabilise the Syrian country, to avoid further civil war, then work collectively with our partners around the world on political protests that would lead to Assad leaving," said Tillerson.

The top US diplomat had said last week that Assad's political future was a matter for the Syrian people to decide, and UN Ambassador Nikki Haley said that defeating the ISIS was the higher priority.

The Obama administration had insisted that Assad could never remain in any post-war Syria, and it supported rebel groups that have tried unsuccessfully to oust him.