China has started constructing the second of four planned aircraft carriers, a top government official said according to media reports on Saturday.

The ship is under construction in the northeastern port of Dalian and will take six years to build, the reports said quoting Wang Min, Communist Party chief for Dalian's Liaoning province.

The country's first aircraft carrier, the Liaoning, was completed in September 2012 in a symbolic milestone for the country's increasingly muscular military.

Another two are in the pipeline, according to Wang, in a projection of power that could be seen as contradicting Beijing's long-stated policy of arming itself strictly for self-defence.

When the Liaoning went into service, Beijing and Tokyo were locked in a territorial row over the Japanese-controlled Senkaku Islands in the East China Sea which China also claims and calls the Diaoyus.

The row continues to simmer, along with other sovereignty disputes with the Philippines and Vietnam.

Early this month a Japanese newspaper said China was overhauling its military structure in order to strengthen its attack capability and secure air and naval superiority in the South China and East China seas.

The Liaoning carrier conducted its maiden mission in the South China Sea in January.

It followed an incident in December in which a US warship was forced to avoid a collision with a Chinese naval vessel, prompting Washington to accuse China of being the aggressor.