The Chinese government has warned Japan to stop an "illegal" trip by politicians to a group of disputed islands under Japanese control, BBC News reported.

A flotilla of some 20 Japanese boats has set sail for the Senkaku (Chinese: Diaoyu) islands and is expected to anchor off them early on Sunday, Aug. 18.

The politicians plan to commemorate Japanese dead in World War II, when Japan occupied eastern China.

But Japan's government has denied them permission to land on the islands.

China says the event will undermine its "territorial sovereignty" and this is the latest move in an escalating dispute over the islands.

On Friday, Japan deported several Chinese activists who had landed there this week.

The islands, also claimed by Taiwan, are close to strategically important shipping lanes, offer rich fishing grounds and are thought to contain oil deposits.