US Secretary of State Rex Tillerson on Thursday said Washington would make an "appropriate response" to Tuesday's suspected chemical attack in Syria, while calling for the ouster of President Bashar al-Assad.

"We are considering an appropriate response (to the) violations of all previous UN resolutions, violations of international norms," he said in televised comments at the Palm Beach International Airport in Florida, where he greeted Chinese President Xi Jinping who was arriving for a summit with US President Donald Trump.

Tillerson did not specify what actions the United States would take.

He added: "Assad's role in the future is uncertain and with the acts that he has taken, it would seem that there would be no role for him to govern the Syrian people."