China on Monday protested at Japan's plan to name several dozen islands near an archipelago at the centre of a territorial dispute between the two countries, state media said.
The plan would include adding the 39 uninhabited islands to maps of Japan by the end of March, including some "surrounding islets" of the Diaoyu Islands, as they are known in China, the official Xinhua news agency reported.
The East China Sea islands -- called Senkaku in Japan, and also claimed by Taiwan -- lie in an area with rich fishing grounds that is also believed to contain oil and gas deposits.
"The Diaoyu Islands and adjacent islets have always been part of Chinese territory, and China exercises its sovereignty over them," foreign ministry spokesman Liu Weimin said in a statement.
"China has already formally protested Japan's plan to name them," he said, adding that such "unilateral action" would be "illegal and invalid."
Since January, four Japanese politicians have visited the islands, sparking Beijing's ire.
In September 2010, relations between Japan and China sank after the seizure of a Chinese fishing boat near the island chain and the arrest of its captain.
The incident sparked the worst tensions between the Asian neighbours in years, with Beijing summoning Tokyo's ambassador and scrapping scheduled talks over joint energy exploration in the East China Sea.
Japan later released the captain, who it maintained had intentionally rammed two Japanese coastguard vessels