The Foreign Ministry said on Friday that Japan's unilateral decision to name China's Diaoyu Islands was "invalid" as it persisted in fueling lingering territorial tensions.

Japan infuriated China and damaged ties two years ago when it said it was "nationalizing" the islands in the East China Sea, which have been part of China's traditional territory and an important fishery for centuries.

The Japanese Cabinet announced on Friday afternoon that the government had given names to "158 unnamed remote islands", including islands in the Diaoyu group.

"The Diaoyu Islands and their affiliated islets are China's inherent territory, and the Chinese side has already given names to all of them," Foreign Ministry spokesman Qin Gang said late on Friday.

China "firmly opposes Japan's move, which infringes on China's territorial sovereignty, and its unilateral measures in this regard are all illegal and invalid. Japan is unable to rewrite the fact that the Diaoyus and affiliated islands belong to China," Qin said.

Gao Hong, a Japan studies research fellow at the Chinese Academy of Social Sciences, said Japan is "once again attempting to justify its grab of China's Diaoyu Islands by telling the world" it is naming the islands, even though they were already named.

As early as Sept 20, 2012, authorities of the Chinese government, including the State Oceanic Administration, standardized the names of the Diaoyus and adjacent islands.

Japan's latest policy campaign to name the so-called "unnamed remote islands" was kicked off in January last year. China has consistently objected.

Tokyo should "keep a sober mind and refrain from making the islands issue a devastating factor" that sinks the relationship, Gao said.

Jin Canrong, a professor of international relations at Renmin University of China in Beijing, said the naming drama on Friday was "staged totally for political considerations" and is "making trouble without the least convincing excuse".

The move came only a week after poll numbers showed that public support for the ruling Cabinet had dropped to a record low.

Jin said the Cabinet "has no fear" of further sabotaging the bilateral relationship with China because "tensions increase public anxiety and boost backing from conservative voters".

In another development, Japan's leading news outlets, including Kyodo News Agency, speculated on Friday that former Japanese Prime Minister Yasuo Fukuda, a frequent guest in China, may have met President Xi Jinping during a secret visit to the country in late July.

Qin of the Foreign Ministry told China Daily in a written response: "Fukuda is chairman of the board of directors of the Boao Forum for Asia, and he often came for working level exchanges with the Chinese side's relevant personnel in charge of the forum's affairs."