The Pentagon is in detailed discussions with the White House about military options on Syria, according to a US official.

This could include grounding aircraft used by Syrian President Bashar al Assad's forces, the official cited by the Reuters news agency added, speaking on condition of anonymity.

Such options would also include using cruise missiles, allowing the US to strike targets without putting piloted aircraft in the skies above Syria.

The White House has said Donald Trump has talked to several leaders about setting up safe zones in the country.

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Speaking on board Air Force One, President Trump said that "something should happen" with Assad, amid widespread condemnation over an alleged gas attack carried out by the Syrian government which left more than 80 people dead.

Mr Trump said what happened was a "disgrace to humanity".

Secretary of State Rex Tillerson said the US was "considering an appropriate response" and added there was "no doubt" President Assad was behind it.

He also called on Russia, a key ally of Assad, to reconsider its support for his regime and said there was "no role" for Assad in Syria's future.

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Moscow has warned against apportioning blame until an investigation has been carried out, but a spokesman for President Vladimir Putin did say earlier that Russia's support for Damascus is "not unconditional".

The Syrian government maintains it did not use chemical weapons, instead blaming opposition fighters for stockpiling chemicals which were hit in an airstrike on a weapons depot.

Discussions are continuing at the UN headquarters in New York on a Security Council resolution that would condemn the attack, although it is not clear if Russia will support it.

Meanwhile, two Republican senators have called on President Trump to take out Syria's air force as part of a quick and forceful response.

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John McCain and Lindsey Graham said Mr Assad "has crossed a line" and "must pay a punitive cost for this horrific attack".

"This is a test of the new administration, but also for our entire country," the pair said in a statement.

"Assad is trying to see what he can get away with."

Senator Graham later said he does not think Mr Trump needs the approval of Congress to use military force in Syria.

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"Hit this guy," he said of Mr Assad. "You've got my full permission."

The official quoted by Reuters did not comment on how likely military action might be or suggest which, if any, options might be recommended.

They did add that US Defense Secretary James Mattis and White House national security adviser H.R. McMaster had already held talks on the issue.

Mr Mattis will presumably discuss the options when he meets with President Trump at his Mar-a-Lago retreat in Florida later, the official said.