WASHINGTON (Reuters) - The Pentagon and the White House are in detailed discussions on military options to respond to a poison gas attack in Syria that killed scores of civilians, and which Washington has blamed on the Syrian government, a U.S. official said on Thursday.

A civil defence member breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib. REUTERS/Ammar Abdullah

Those options include things like grounding aircraft used by Syrian President Bashar al-Assad’s forces, the official added, speaking on condition of anonymity.

Such options also include use of cruise missiles - which allow the United States to strike targets without putting piloted aircraft in the skies above Syria.

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

But the official added that U.S. Defense Secretary Jim Mattis and White House national security adviser H.R. McMaster had already been engaged in extensive talks on the matter.

Mattis will presumably discuss the options when he meets with President Donald Trump at his Mar-a-Lago retreat in Florida, the official said. Mattis is due in Florida later on Thursday as part of a scheduled trip tied to the visit of Chinese President Xi Jinping.

Trump said on Thursday that “something should happen” with Assad after the attack, but stopped short of saying he should leave office. U.S. Secretary of State Rex Tillerson said however, there was no role for Assad in Syria in the future.

“I think what Assad did is terrible,” Trump told reporters traveling with him on Air Force One en route to Florida. “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 said.

His accusations against Assad put him directly at odds with Moscow, the Syrian president’s principal backer.

POSSIBLE NEW FRONT

Any U.S. action against Syria’s government would open a new front in Syria’s fighting, with consequences that are difficult to foresee.

Entering into such a confrontation might complicate the fight against Islamic State - a group seen to directly threaten the West - and potentially draw in Russia.

Among other valid military targets in Syria would be Syrian military airfields, air defenses and other types of Syrian military installations. The official played down the idea that Russian military infrastructure might be a target.

Although any chemical weapons storage facilities would also be valid military targets, it was not immediately clear how much intelligence the United States had collected on where Assad might be storing the kinds of nerve agent it believes was used in the most recent attack.

The U.S. military, which has deployed around 1,000 troops in Syria, who are there without the invitation or consent of Syria’s government, has long said its singular focus in Syria has been on the war against Islamic State.

That conflict is reaching a critical point as U.S.-backed forces isolate the city of Raqqa - the militants’ de facto capital - ahead of an eventual assault.