TOKYO (Reuters) - Japan is set to clarify the ownership of 280 remote islands within its territorial waters and register them as national assets, a move that could rile China and South Korea, currently engaged in territorial disputes with Tokyo.

The move for the government to survey the islands and claim those with no apparent owners was announced this week and continues a plan first begun five years ago, an official at the Oceanic Policy and Territorial Issues secretariat said.

"Basically the idea is to register these islands as national assets," the official added.

He said the location of the islands remained unclear until the survey was completed, but they were all within Japanese territorial waters and the boundaries of the country's exclusive economic zone (EEZ) would not change.

Since the plan kicked off, Japan has nationalised some 99 remote islands with no apparent owner. That figure is separate from the number now targeted for survey.

Ties between Japan and China have been strained due to a simmering row over ownership of a group of uninhabited islands in the East China Sea, while Japan and South Korea are locked in a territorial row over a different set of islands.

Japanese Prime Minister Shinzo Abe's Dec 26 visit to Tokyo's Yasukuni Shrine, where Japanese leaders convicted as war criminals are enshrined along with war dead, infuriated China and South Korea and stoked concern from the United States, a key ally.