The planning also included updates on the potential use of cruise missiles, which would not require fighter pilots to enter Syrian airspace.

But the official cautioned the steps were taken "to give the president a current and comprehensive range of choices" and that no decisions were made at a national security meeting on Thursday at the White House.

The official said there are certain static targets, like government buildings and military installations, but that forces and equipment of Syrian President Bashar al-Assad "continue to move" and thus require flexibility in planning.

Defense Secretary Chuck Hagel said Friday "a very quick assessment" of the allegations is important because if the Assad government did use chemical weapons, "there may be another attack coming," Reuters reported from aboard the military aircraft carrying Hagel to Malaysia.

Hagel told reporters on the plane that the military was responsible for "positioning our assets to be able to carry out different options," but he said that did not necessarily mean forces or assets had been moved, according to Reuters.

Military planners last conducted a major update of options for Syria in April, in response to bipartisan pressure from members of Congress.

"The plans are constantly being reviewed and tweaked," the official said, adding that the latest update represented a more comprehensive review of airstrike target lists.

Late on Friday, a defense official said the United States had added a Navy destroyer to the eastern Mediterranean fleet.

The USS Ramage arrived to replace the USS Mahan, but the Mahan is going to stay around a bit longer, so temporarily there will be four. The USS Gravelly and USS Barry remain.

These ships are all equipped with the Tomahawk Land Attack Missiles, a long-range subsonic cruise missile used to attack land targets.