Tokyo: Japan's Defence Minister called on China on Friday to explain why it needs an aircraft carrier, after Beijing sparked increased concerns over its military expansion by starting sea trials for the vessel.

"As an aircraft carrier, it is of a highly maneuverable and offensive nature. We want China to explain the reasons why it needs it," Toshimi Kitazawa told reporters.

"There is no doubt that it will have a big impact on the region," he added.

China put the revamped Soviet-built aircraft carrier Varyag to sea on Wednesday, prompting the United States to call for an explanation.

Beijing has sought to play down the vessel's capability, saying it will mainly be used for training and "research".

In its annual defence report last week, Japan expressed concern over China's growing assertiveness and widening naval reach in nearby waters and the Pacific and over what it called the "opaqueness" of Beijing's military budget.

China criticised the report as "irresponsible", insisting its drive to modernise its forces was entirely defensive.