"The Diaoyu Islands and adjacent islets have been an inherent territory of China since ancient times, over which China has indisputable sovereignty," ministry spokesman Hong Lei said, stressing that China is firmly determined to defend its sovereignty of the islands.
According to Kyodo News, the four illegal intruders were all members of the Ishigaki city assembly in Okinawa Prefecture.
A patrol ship of Japan's coast guard confirmed Hitoshi Nakama and two others landed first at around 9:30 am local time, followed 20 minutes later by Tadashi Nakamine, the fourth member. All four assembly members left at around 11:55 am.
This was not the first time Nakama, one of the four city councilors, had set foot on the Diaoyu Islands.
Kyodo News said he went to one of the islets in December 2010, not long after the collision between a Chinese fishing trawler and Japanese coast guard ships.
"Though the landings on the islands were just the individual actions of some city councilors, Japan has still got off on the wrong foot at the very beginning of this year, the 40th anniversary of normalization of diplomatic ties between China and Japan," Liu Jiangyong, an expert on Japan studies at Tsinghua University, told China Daily.
Although the Diaoyu Islands are rocky and uninhabited, they remain one of the thorniest issues between China and Japan.
The Japanese government had said it prohibits its citizens from landing on the Diaoyu Islands to avoid conflicts with China.
Kyodo News also reported that members of the World Chinese Alliance in Defense of the Diaoyu Islands, based in Hong Kong, planned to leave for the Diaoyu Islands on Tuesday, but their move was stopped by the Hong Kong Special Administrative Region government.
