Japan's Ishigaki city assembly on Monday passed a bill to rename an administrative area covering a group of islands in the East China Sea, immediately triggering backlash from China and Taiwan, both of which also claim the uninhabited islets as their own.

The assembly changed the name of the southern Japan area containing the Senkaku Islands from "Tonoshiro" to "Tonoshiro Senkaku," which both Beijing and Taipei see as an attempt to cement Tokyo's claim by inserting the Japanese name "Senkaku."

The city in Okinawa Prefecture says that the renaming takes effect on Oct. 1 and it is aimed at resolving administrative confusion between a locale in downtown Ishigaki, which shares the name "Tonoshiro" with the isles.

File photo from September 2012 shows the Senkaku Islands. (Kyodo)

The municipal assembly was able to make the change of the area as it falls under Ishigaki's administrative authority.

China immediately slammed the latest decision by Japan's city, saying the islands and its affiliated islets are the country's "inherent territory" and its determination to safeguard territorial sovereignty is "unswerving."

"The adoption of the so-called name change bill by Japan is a serious challenge to China's territorial sovereignty. It is illegal and invalid," Foreign Ministry spokesman Zhao Lijian told reporters. Beijing claims sovereignty over the isles, calling them Diaoyu.

The renaming "cannot change the fact that the Diaoyu Islands belong to China," Zhao said, adding that Beijing has lodged "solemn representations" to Tokyo through diplomatic channels.

Taiwan, meanwhile, urged Japan to exercise self-restraint so as not to undermine bilateral ties as well as peace and stability in the East China Sea, pledging to continue handling the matter through peaceful and reasonable means.

"Any unilateral action does not change the fact that the Tiaoyutai Islands are the sovereign territory of the Republic of China," Presidential Office spokesman Xavier Chang said, using Taiwan's name for the islets. The Republic of China is Taiwan's official name.

Taipei also said Beijing's ships have disrupted fishing activities in the contested waters for an extended period, prodding the city to pass the bill. It called on the mainland government to try to resolve the territorial spats in a peaceful manner.

On Monday, four China Coast Guard vessels operated in the contiguous zone near the Senkakus. Chinese ships have been spotted in the area for the 70th consecutive day, the longest since September 2012.

Ishigaki Mayor Yoshitaka Nakayama submitted the bill for the change after Japanese fishing boats were chased by Chinese patrol boats in Japanese territorial waters near the islands in early May, fanning speculation that the move is a response.

Nakayama, however, has denied the change is designed to bolster Japan's claim over the area, telling the city assembly it is "merely intended to streamline administrative work."

Tensions over the islets escalated after Japan brought them under state control in September 2012.

Taiwan's northeastern county of Yilan has adopted a proposal to rename the islands from "Tiaoyutai" to "Toucheng Tiaoyutai" to include the name of a local township.

In recent years, Sino-Japanese relations have been improving by effectively shelving the territorial issue in the East China Sea.

Tokyo cut diplomatic ties with Taipei and established them with Beijing in 1972, but Japan and Taiwan have boosted private-sector-led economic relations and people-to-people exchanges in wide areas, ranging from education, art and sports to health care.

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