Taiwan summoned Japan's representative Sunday to protest against the "provocative" act by a group of Japanese nationalists who landed on an island at the heart of a territorial dispute.
A group of about 150 Japanese right-wing activists and lawmakers sailed to the disputed islands in the East China Sea and some of them early Sunday set foot on one of the islands, which are administered by Tokyo and also claimed by China and Taiwan.
Taiwanese Foreign Minister Timothy Yang immediately summoned Tadashi Imai, Japan's de facto ambassador in the absence of diplomatic links.
"The provocative measure has added fuel to the tensions which are mounting in the East China Sea area," the foreign ministry said in a statement.
Last month, coastguard vessels from Taiwan and Japan "bumped into" each other in waters near the disputed island chain as the Taiwanese vessel was escorting activists to the area.
"The Diaoyutai island group belongs to the Republic of China (Taiwan's official title), given from geographical, historical and international law point of view. The Republic of China's ownership is irrefutable," the statement said.
"The unilateral move and argument by the Japanese group of people cannot possibly change the reality and the Japanese government must immediately halt the action," it said.
The landing comes just days after Tokyo deported pro-Beijing 14 protesters from Hong Kong and Macau who had landed on the island.
The uninhabited outcrops, known as the Senkaku islands in Japan, were the scene of a spat in late 2010 when Japan arrested a Chinese trawlerman who had rammed two of its coast guard vessels.
Japan officially recognises Beijing rather than Taipei but has maintained close trade and cultural ties with Taiwan, which was its colony between 1895 and 1945.