Depending on security conditions, it could range from simply providing perimeter security for Americans boarding military aircraft to a more dangerous mission where military aircraft quickly land at a secret gathering spot under the cover of darkness to minimize exposure to hostile combat conditions, officials said.

It is the State Department that decides what course of action should be taken to protect its embassy staff and dependents. Sometimes the State Department is able to contract charters or fly personnel out on passenger aircraft, but if security deteriorates to a point where that is not possible, then the Pentagon is asked to provide U.S. military assets that could be used to help evacuate staff, officials said.

The Marines and 10 aircraft have been stationed at the Naval Air Station in Sigonella, Sicily, as a precautionary move, Pentagon Press Secretary Rear Admiral John Kirby said at a briefing today.

The Marines are part of the Crisis Response force of 1,000 Marines based in Moron, Spain, that are available for service in North Africa that was activated in the wake of the Benghazi attack in September 2012. This is the fourth time in the last 12 months that they have been deployed to Sigonella, officials said.