Japan plans to dispatch its largest warship on a three-month tour through the South China Sea in May, three sources said, in its biggest show of naval force in the region since World War II.

The Izumo helicopter carrier, commissioned only two years ago, will make stops in Singapore, Indonesia, the Philippines, and Sri Lanka before joining the Malabar joint naval exercise with Indian and US naval vessels in the Indian Ocean in July, the sources told Reuters news agency.

The carrier will then return to Japan in August.

"The aim is to test the capability of the Izumo by sending it out on an extended mission," said one of the sources with knowledge of the plan.

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"It will train with the US Navy in the South China Sea," he added, asking not to be identified because he is not authorised to talk to the media.

The 249-metre-long Izumo is as large as Japan's World War II-era carriers and can operate up to nine helicopters.

It resembles the amphibious assault carriers used by US Marines, but lacks their well deck for launching landing craft and other vessels.

A spokesman for Japan's Maritime Self Defence Force declined to comment.

China claims almost all the disputed waters in the South China Sea and its growing military presence has fuelled concern in Japan and the West, with the US holding regular air and naval patrols to ensure freedom of navigation.

Taiwan, Malaysia, Vietnam, the Philippines, and Brunei also claim parts of the sea, which has rich fishing grounds, oil and gas deposits, and through which about $5 trillion in global sea trade passes each year.

Japan does not have any claim to the waters, but has a separate maritime dispute with China in the East China Sea.