Western air strikes in Libya entered its sixth day on Thursday as the world questioned the motive behind.

Though the air raids were claimed as a way to safeguard the UN resolution imposing a no-fly zone in the North African country, the real intent were in doubts.

U.S. State Secretary Hillary Clinton on Wednesday urged Libyan leader Muammar Gaddafi to leave power, saying "the quickest way for him to end this is to actually serve the Libyan people by leaving."

Airstrikes target ground forces

According to the U.S. Department of Defense, a total of 336 sorties have been flown by the Western military, including 108 air strikes since the start of the operations against Libya.

Earlier reports said that with no Libyan planes taking to the sky in the past 24 hours due to its own air force disarray, the coalition military has been targeting Libyan ground forces so as to further clamp down on Gaddafi.

On Wednesday night, several targets in the Tajoura district in Tripoli's eastern suburb were attacked by three rounds of bombing, and a military engineering institute was hit and caught fire.

Several cars parking nearby or passing by were also struck by the bombing and people inside were injured, local residents told Xinhua.

Meanwhile, Libyan media said Wednesday that "a large number" of civilians have been killed in east Tripoli by Western air strikes, which is yet to be confirmed by the authorities. But a spokesperson from the Libyan government denied earlier reports saying that Khamis, a son of Gaddafi, was killed in the first round of West strikes on Saturday.

Earlier in the day, Navy Adm. Gary Roughead, chief of naval operations of the U.S. Navy has said the military campaign over Libya is not "especially costly" from a funding standpoint, as there were already deployments in the Mediterranean Sea before Libya's situation flared up.

Roughead's remarks came hours after the U.S. Navy dispatched three more warships to the Mediterranean, which carried hundreds of Marines, a team of surgeons and a helicopter sea combat squadron.