Enforcing a no-fly zone over Libya would first require bombing the north African nation's air defense systems, top US commander General James Mattis warned today.
A no-fly zone would require removing "the air defense capability first," Mattis, the head of Central Command, told a Senate hearing.
"It would be a military operation," the general said.
The United States and its allies are weighing possible military action, including a no-fly zone, to stop Libya's Muammar Gaddafi from using his forces to crush mounting opposition.
Although Gaddafi's military is badly outgunned by US and NATO aircraft, the regime has dozens of surface-to-air missiles that could shoot down allied warplanes.
US military doctrine usually requires knocking out an adversary's air defense missiles and radar any time air power is used.
The general's comments came as the American military moved naval and air forces near Libya for possible joint NATO military action, including a warship with hundreds of Marines.
The USS Kearsarge, an amphibious assault ship accompanied by two other naval vessels, was expected to pass through the Suez Canal soon from the Red Sea, two defence officials, who spoke on condition of anonymity, told AFP.
The Kearsarge amphibious ready group, with about 800 marines, a fleet of helicopters and medical facilities, could support humanitarian efforts as well as military operations.
US military leaders are preparing a range of options for President Barack Obama and holding discussions with their European counterparts, but the likelihood of military intervention remained unclear, one of the defense officials said.
"I think it (the advice) goes from everything from a show of force to something more involved," the official said, adding: "The president has made no decisions about the use of the military."
Analysts say a symbolic show of force off the coast of Libya in and of itself could increase pressure on Gaddafi but the official said the deployment of naval and air forces near Libya was not an empty gesture.
"There are sailors on ships heading that way, it's real."
An American aircraft carrier, the USS Enterprise -- which has fighter jets that could enforce a possible no-fly zone -- could also be called upon for the Libya crisis.
The carrier is currently in the north of the Red Sea near the mouth of Suez Canal, according to the US Navy's website.
A no-fly zone would likely require large numbers of aircraft, with US bases in southern Italy providing a staging area for operations.
The United States and its partners might have to ask other countries in the region, including Egypt or Tunisia, for permission to use their airfields, analysts say.
The West heaped pressure on Gaddafi today after loyalists tried to retake a key city near the capital following a show of defiance by the veteran leader.