China's first aircraft carrier has returned to the sea for its second set of trials, amid heightened concerns about the country's maritime claims.

The defense ministry said the carrier left port Tuesday to conduct research experiments.

China bought the former Soviet vessel from Ukraine in 1998, and the refitted ship completed a first set of sea trials in August.

The United States has expressed concern about China's lack of transparency regarding the carrier, but Chinese officials say the vessel is not a threat to anyone.

China's push to build up its navy has been accompanied by a major diplomatic dispute with Japan over islands in the East China Sea, and moves to disrupt oil and gas exploration by Vietnam and the Philippines in disputed areas of the South China Sea.

Chinese naval commanders have long worried about their access to the Pacific Ocean. Japan permits free navigation through an island chain that runs from South Korea to Taiwan, however it keeps a close watch on Chinese ship movements.

Japan's defense ministry said last week that six Chinese naval vessels crossed between two of its islands in the western Pacific.