Possible military choices range from limited one-off missile strikes from ships - one of the less complicated scenarios - to bolder operations like carving out no-fly safe zones.

One of the most politically unpalatable possibilities envisions sending tens of thousands of U.S. forces to help secure Syrian chemical weapons.

Obama has so far opposed limited steps, like arming anti-government rebels, but pressure to deepen U.S. involvement in Syria's civil war has grown since Thursday's White House announcement that President Bashar al-Assad likely used chemical weapons.

After fighting wars in Afghanistan and Iraq, the Pentagon is wary of U.S. involvement in Syria. The president's top uniformed military adviser, General Martin Dempsey, said last month he could not see a U.S. military option with an "understandable outcome" there.

"There's a lot of analysis to be done before reaching any major decisions that would push U.S. policy more in the direction of military options," a senior U.S. official told Reuters.

That caution is understandable, given the experience of Iraq where the United States went to war based on bad intelligence about weapons of mass destruction. The Pentagon has made repeated warnings of the enormous risks and limitations of using American military might in Syria's civil war.