Additional military options?
• Targeting Syria's air defenses with Patriot missile batteries in Turkey as part of a "no-fly" response, NATO has said. But Turkey only allowed the missiles there for defensive purposes and NATO nations would have to agree before they could be used otherwise.
• Airstrikes: The U.S. military has enough air power in the region to take action against Syria, according to officials. That includes fighter jets and bombers spread out across air bases in the Middle East and nearby aircraft carriers. The Navy also has warships equipped with Tomahawk missiles, which could be used to hit chemical weapons supplies.
But such strikes pose a danger of releasing chemical agents into the air around civilian populations in Syria, U.S. officials told CNN in March.
Complicating military involvement
What complicates any U.S. military support for the opposition is that many of the rebel fighters are militants with pro-al Qaeda sympathies, the same stripe of militants America has battled in Iraq and Afghanistan.
In April, Hagel cautioned lawmakers on the Senate Armed Services Committee about the difficulties surrounding any direct U.S. military action in Syria.
"It could embroil the United States in a significant, lengthy and uncertain military commitment," he said.
He called military intervention "an option but an option of last resort."
Joint Chiefs Chairman Gen. Martin Dempsey said the United States could send troops to Syria if al-Assad's government fell, if needed, to secure chemical weapons.