WASHINGTON - President Donald Trump has told some members of Congress that he is considering military action in Syria in retaliation for this week's chemical attack, according to a CNN report which quoted a source familiar with the calls.

The source said the President recognises the seriousness of the situation and had not firmly decided to go ahead with military action.

He is said to be discussing possible actions with Defence Secretary James Mattis, said CNN. Trump is relying on the judgment of Mattis, according to the source.

US officials told CNN the Pentagon had long-standing options to strike Syria's chemical weapons capability and had presented those options to the administration.

The sources stressed a decision has not been made.