US President Donald Trump has hinted at possible military action against Syrian President Bashar al-Assad as his administration considers retaliatory options after a chemical weapons attack killed more than 80 people.

"I think what happened in Syria is a disgrace to humanity and he's there, and I guess he's running things, so something should happen," Trump told reporters on Air Force One, en route to meet China's President Xi Jinping at a Florida summit on Thursday.

While Trump stopped short of calling on Assad to leave office, the comments were his strongest suggestion yet that the United States may be edging towards a stronger stance against Assad.

His stance was underlined by Secretary of State Rex Tillerson, who told reporters: "There's no role for [Assad] to govern the Syrian people".

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Tillerson, who spoke almost simultaneously after greeting Xi in West Palm Beach, said the US was evaluating an appropriate response.

"The process by which Assad would leave is something that will require an international community effort," Tillerson said, adding there needs to be a balance between defeating the Islamic State of Iraq and the Levant (ISIL) and stabilising Syria to prevent the war from escalating.

Tillerson also issued a warning to Russia, a key ally of Damascus, that its support of the Assad government is something it should "consider carefully".

The poison gas attack on the rebel-held town of Khan Sheikhoun in Idlib province killed at least 86 people, including 27 children, according to the UK-based Syrian Observatory for Human Rights.

Turkey said samples from victims of Tuesday's attack indicate they were exposed to sarin, a highly toxic nerve agent.

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The World Health Organization and Doctors Without Borders also reported that victims from the attack showed symptoms consistent with the use of a nerve agent such as sarin.

The Syrian government rejected the accusations, and Moscow warned against apportioning blame until an investigation has been carried out.

Trump has until now focused his Syria policy almost exclusively on defeating ISIL in northern Syria, where US special forces are working with Arab and Kurdish armed groups.

Only last week, the US Ambassador to the United Nations Nikki Haley said the US diplomatic policy on Syria was no longer focused on forcing Assad to leave power.

But Trump said on Wednesday the gas attack had caused him to reconsider his opinion of Assad, adding it crossed "many, many lines" - not just the "red line" of chemical weapons use that former president Barack Obama once set as an ultimatum for the Assad government.

An anonymous US official told Reuters news agency on Thursday that options being discussed by the Pentagon and White House could include grounding aircraft used by Assad's forces.

Others could include the use of cruise missiles, allowing the US to strike targets without putting piloted aircraft in the skies above Syria.

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Turkish President Recep Tayyip Erdogan said on Thursday he hoped Trump would take military action, Turkey's state-run Anadolu Agency reported.

Erdogan said Turkey would be prepared to do "whatever falls on us" to support possible military action.

Jessica Ashooh, the deputy director of the Middle East Strategy Task Force at the Atlantic Council, a foreign policy think-tank in Washington DC, said every president is tested early on and it appears that Syria has become the test for Trump and his administration.

"What he does in this situation is going to colour how the world views his presidency and it's going to influence what countries like Russia or North Korea believe they can or cannot get away with," she told Al Jazeera.

If confirmed, the chemical attack will be the deadliest since sarin gas killed hundreds of civilians in Ghouta near the capital, Damascus, in August 2013.

Western states said the Syrian government was responsible for the 2013 attack. Damascus blamed rebels.