Beijing has reiterated China's sovereignty over the Diaoyu Islands on Tuesday, following the governor of Tokyo's sudden announcement of a plan to buy part of the islands.

Shintaro Ishihara, Tokyo's governor, said in Washington on Monday that his city prefecture is negotiating with the so-called "owner" of the islands, with the aim of "buying them by the end of this year", Kyodo News Agency reported.

Ishihara said in a speech to the conservative Heritage Foundation that he had begun negotiations to purchase three islets of the Diaoyu Islands, which Ishihara claimed are "owned by a Japanese family".

The Diaoyu Islands and its affiliated islets have been China's territory since ancient times, and China has undisputable sovereignty over the islands, Chinese Foreign Ministry spokesman Liu Weimin told reporters on Tuesday.

"Any unilateral action taken by the Japanese side over the Diaoyu Islands and its affiliated islets is illegal and invalid, and will not change the fact that these islands belong to China," Liu said.

But Japan's government also seems to have been taken by surprise by the governor's remarks.

"I received no report about Ishihara's remark before it came out," Japanese Foreign Minister Gemba Koichiro told a news conference in Tokyo on Tuesday.

The relevant prefectural department in Tokyo said they "had not received a detailed request from Ishihara so far".

Concerns surfaced after Kyodo said the sudden announcement would "inevitably draw fire" from China. Japan's Jiji Press News Agency also warned that the remarks would "give rise to a new round of frictions between both countries".

Attempts by the Japanese government or a "private owner" to lay claim to China's territory are unacceptable and doomed to fail, analysts said.

"Ishihara's plan has been completely wrong from the very beginning, and the sudden remark serves his own political interests in campaigning for his new party," said Huo Jiangang, an expert on Japan studies at the China Institutes of Contemporary International Relations.

Tokyo's disputes with Beijing over the sovereignty of the Diaoyu Islands have given rise to frequent friction between the two countries and have cast a shadow over bilateral relations.

Japan has repeatedly taken measures since the 1990s aimed at putting the Diaoyu Islands under its control.

The sudden remark was also made to cater to nationalist sentiments among the Japanese public, Huo added.

Tokyo's prefectural government received phone calls from some civilians on Tuesday morning in support of Ishihara's plan, according to Japan's Sankei Shimbun newspaper.

Ishihara, elected for a fourth term as governor a year ago, was described as an "outspoken and hawkish" politician by Japanese media.

He was forced to apologize last year for suggesting that the March 11 earthquake and tsunami were "divine punishment" for the egoism of the Japanese people.

This year marks the 40th anniversary of the normalization of China-Japan diplomatic relations.