Establishing a no-fly zone over Libya would require widespread bombings to destroy Libyan air defenses first, a top U.S. defense official said on Wednesday.

It's a big operation in a big country, Secretary of Defense Robert Gates told a House subcommittee.

Gates, re-iterating comments made by other military officials this week, said enforcing a no-fly zone would be an extensive military operation.

Let's just call a spade a spade, he said.

A no-fly zone begins with an attack on Libya to destroy the air defenses. That's the way you do a no-fly zone. And then you can fly planes around the country and not worry about our guys being shot down. But that's the way it starts.

[T]he no-fly zone is an option that is being actively considered, White House spokesman Jay Carney said on Tuesday.

The United States has experience with enforcing no-fly zones. It previously one over Iraq for more than a decade.