BEIJING >>Three Japanese detained after allegedly entering a restricted Chinese military zone were released Thursday, but one was still being held amid simmering tensions after ships collided near disputed islands.

The three were freed after admitting to violating Chinese law but the fourth, identified as Sadamu Takahashi, remained under house arrest and was being investigated for illegally videotaping military targets, China’s official Xinhua News Agency reported.

The four were detained outside the northern city of Shijiazhuang on Sept. 21 amid a bitter territorial dispute between the countries triggered by a Sept. 7 collision between a Chinese fishing trawler and two Japanese patrol boats near disputed islands in the East China Sea.

Japan released the fishing boat captain over the weekend and said China needs to resolve the case of the four as the first step toward repairing ties.

China’s Foreign Ministry has denied any link between the detentions and the islands incident.

The arrests have been widely interpreted as one of a series of retaliatory moves aimed at pressuring Japan into acceding to Beijing’s demands over the islands issue.

Beijing had suspended provincial and ministerial-level contacts with Japan, along with talks on mutual development of gas-and-oil deposits in the East China Sea. China also reportedly suspended exports of rare earth minerals crucial to Japan’s high-tech sector and stepped-up customs inspections, slowing trade between Japan and its biggest export market.

In Tokyo, meanwhile, Prime Minister Naoto Kan called China’s response to the incident “extremely problematic,” and reasserted Japan’s claims to the disputed islands, known as Diaoyutai or Diaoyudao in Chinese and Senkaku in Japanese.

Japan “needs to make clear its stance that the Senkaku Islands are an integral part of Japanese territory,” Kan said at a parliamentary session convened specifically to discuss the collision case.

Foreign Minister Seiji Maehara confirmed the three were released, and said Tokyo was seeking an “early resolution” to the continued detention of Takahashi.

The four detained are employees of Fujita Corp., a Tokyo-based construction and urban redevelopment company, which has said the men were in China working to prepare a bid for a project to dispose of chemical weapons abandoned in China by the Japanese military at the end of World War II.

The company has identified the other three as Yoshiro Sasaki, 44, Hiroshi Hashimoto, 39, and Junichi Iguchi, 59.

The latest confrontation plunged relations between the sides to their lowest level since the 2001-2006 term of former Prime Minister Junichiro Koizumi, whose repeated visits to a war shrine in Japan enraged China and sparked a wave of violent anti-Japanese protests across the country.

The spat – and China’s unusually strong response – also raised questions about cooperation between the Asian powers at international meetings. Kan and Chinese Premier Wen Jiabao essentially ignored each other at a recent gathering at the United Nations and have no plans to meet at a major Asia-Europe forum in Belgium this weekend.

Despite that, both sides now appeared to be moving to contain the fallout, with China’s communist leaders not wishing to further stoke public anger that risks morphing into a genuine protest movement, said David Zweig, director of the Center on China’s Transnational Relations at Hong Kong University of Science and Technology.

“China never really wanted to see it get out of hand. It doesn’t like to see foreign policy go to the street,” he said.

Beijing is also seen as not wishing to poison relations with Kan’s new government, which took office less than four months ago, or strengthen the hand of hard-liners in Tokyo.

-Associated Press writer Shino Yuasa in Tokyo contributed to this report.