US Secretary of State calls for Assad's ouster

"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."

Trump also denounced Assad calling him responsible for a "terrible," "egregious" crime and suggesting that the United States could be poised to take action over the attack.

At least 86 people were killed early on Tuesday in rebel-held Khan Sheikhun, and dozens more treated for convulsions, breathing problems and foaming at the mouth.

"What Assad did is terrible. What happened in Syria is truly one of the egregious crimes," Trump said on Thursday while travelling to Florida on board Air Force One.

"I think what happened in Syria is a disgrace to humanity, and he's there, and I guess he's running things, so I guess something should happen."

He also warned that Assad's regime had crossed a line, calling the strike an "affront to humanity".

The statements come as the Pentagon presents a range of possible military options the United States could take in response to the attack.

Options include strikes to ground the Syrian air force, an official said, speaking on condition of anonymity.

Pentagon chief Jim Mattis is presenting the options to President Donald Trump and administration officials in response to White House requests, the official added.

Mattis had been communicating extensively with Trump's National Security Advisor H.R. McMaster, the official said, stressing that no decisions had been taken.

Agencies contributed to this report