U.S. Defense Secretary Chuck Hagel visited China's sole aircraft carrier Monday in an unprecedented opening by normally secretive Beijing to a potent symbol of its military buildup.

A U.S. official said the visit to the carrier, the Liaoning, at a port in the northern city of Qingdao, lasted about two hours. No other details were immediately available.

The official believed Hagel was the first official visitor from outside China to be allowed on board the Liaoning, although that could not be immediately confirmed.