TOKYO (Reuters) - Japan has filed a protest to Beijing after the discovery that China installed radar equipment in a gas exploration platform close to disputed waters in the East China Sea, a Japanese foreign ministry spokesman said on Sunday.

Japan fears that the radar, a type commonly found on patrol ships and not necessary for gas field development, could be a sign that China intends to use gas exploration platforms in the disputed waters as military stations, Japanese media said.

Also on Sunday, a record number of Chinese coastguard and other government ships entered areas of waters just outside what Japan considers its territorial waters around a group of contested East China Sea islets, further stoking tensions.

The entry of 13 Chinese government vessels into “contiguous waters”, which countries can police for customs and immigration violations, took place despite Japan’s repeated protests over recent, smaller-scale entries.

According to the spokesman, Japan discovered the radar in late June and issued a protest on Friday through its embassy in China, urging Beijing to explain the purpose.

Japan has been calling on China to halt construction of oil-and-gas exploration platforms in the East China Sea, accusing it of unilateral development despite a 2008 agreement to maintain cooperation on resources development in the area, where no official border between them has been drawn.

The latest protests adds to bilateral tensions between the two Asian neighbours over territorial claims and comes less than a month after an arbitration court in The Hague invalidated China’s sweeping claims in the South China Sea.

Ties between China and Japan, the world’s second- and third-largest economies, have been plagued by the row over the islets controlled by Tokyo but also claimed by Beijing, a legacy of Japan’s wartime aggression and regional rivalry.