With the security situation in Libya deteriorating daily, the Pentagon has placed 250 Marines and aircraft in Sicily as a precautionary move should the State Department request the evacuation of American staff from the embassy in Tripoli, Libya, military officials said today.

But if the Marines are called in, what would an evacuation look like?

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.

Kirby said the Marines' move to Sicily was a "prudent measure" made by Gen. David Rodriguez, commander of U.S. Africa Command, so they could be closer to Libya if their services are needed. Kirby stressed that the State Department has not requested U.S. military operations in Libya and there has been no change in embassy operations in Tripoli.

Any use of U.S. military assets in Libya would require the consent of the Libyan government, officials said.

If the U.S. military is asked to facilitate the evacuation of embassy staff from Tripoli, a lot will depend on the security situation on the ground. The Marine aircraft deployed to Sicily include eight MV-22 Osprey tilt-rotor aircraft that can land and take off like a helicopter then fly like a fixed-wing aircraft. They are ideal for the quick landings and take-offs needed in hostile conditions.

In a non-hostile security environment, U.S. military forces will work with local security forces to provide perimeter security at runways where American civilians have gathered to board U.S. military aircraft, officials said.

In hostile environments, security and secrecy are at a premium to ensure the safety of American evacuees, officials said. Evacuees will be told to gather at a pre-determined location where U.S. military assets will provide perimeter security after arriving on aircraft that will make quick landings to board evacuees.

The security crisis in South Sudan in December and January provides a good illustration or how the U.S. military conducts evacuation operations under both types of security conditions.

In late December, a large group of American nationals were evacuated aboard U.S. military aircraft that arrived at the airport in the South Sudanese capital of Juba. By early January the security situation in Juba had deteriorated so much that the evacuation of 20 embassy staffers was conducted under more stressful conditions. A force of Marines -from the unit based in Spain - provided perimeter security at the airport while the embassy staffers boarded a C-130 military transport aircraft. An additional force of 45 U.S. Army soldiers from a crisis response team based in Djibouti also arrived in Juba to enhance security conditions at the U.S. embassy as the violence escalated, officials said.

But even a stable environment can turn hostile quickly, as happened in late December during another evacuation attempt in South Sudan. On Dec. 21, a mission to evacuate American civilians working at a besieged refugee camp was aborted after the three CV-22 Osprey aircraft that were to take the civilians took ground fire, wounding four Navy SEALS aboard, Pentagon officials said.

The aircraft were headed towards what they thought was a secure gathering point because rival fighting forces had been notified of their arrival, but word did not filter down to all of the fighters who then fired at the incoming aircraft, officials said.