The US Navy reportedly has three submarines outfitted with Tomahawk missiles in the Mediterranean in preparation for operations against Libya.
A US official told Reuters the submarines are equipped with Tomahawk missiles that can cripple aircraft or anti-aircraft defenses.
Earlier, allied warplanes went into action to stop Libyan leader Colonel Gaddafi's forces attacking his own people.
France said its jets have destroyed a number of military vehicles.
It is the first reported offensive military action against the dictator's troops since the UN authorised operations to protect civilians in Libya.
Gaddafi's troops pushed into the outskirts of Benghazi after a unilateral ceasefire declared by his government failed to materialise, prompting leaders meeting in Paris to announce the start of military intervention.