U.S. Defense Secretary Leon Panetta says the NATO mission in Libya is an example of why the alliance matters, and shows that it can mount an effective campaign with Europe playing a lead role.

His comments came Wednesday in Brussels, where NATO defense ministers are gathering to discuss the situation in Libya and other operations.

Panetta said Tuesday the NATO air campaign over Libya cannot end and the political process cannot begin until former leader Moammar Gadhafi's hometown of Sirte and other areas are taken. He said not knowing Mr. Gadhafi's whereabouts also makes it difficult for the alliance to decide when to end its air operation and allow the interim government to move on to other issues.

Libyans poured out of Sirte Tuesday, after three weeks of brutal shelling and gunbattles finally forced them to flee through hostile interim government checkpoints.

Cars piled high with possessions and carrying terrified, hungry families crawled out of the strategic coastal city to have their belongings searched and identities checked by suspicious National Transitional Council forces.

NTC fighters eased their shelling of the town Tuesday, allowing residents to escape. Neither the fleeing Sirte families nor the revolutionary fighters searching them hid their mutual distrust, although most civilians said they were well treated and some were given food and water.

NTC leaders have vowed to step down after Sirte is secured and the country's liberation declared.

Interim Prime Minister Mahmoud Jibril said Monday that NTC control of Sirte would ensure that all of the country's sea, land and air entry ports are secure. He acknowledged that fighting will continue in a number of pro-Gadhafi pockets deep in the southern desert, including Bani Walid.

Jibril and NTC head Mustafa Abdel Jalil already had pledged to take no further part in the country's future government, as required by the country's interim constitution. The pledge was intended to reassure the public it will not suffer under another dictatorship.