Tokyo, Japan (CNN) -- Four Japanese nationals have been arrested in China, and are being investigated for entering a military zone without authorization and videotaping military targets, Chinese state media said. The arrests come amid the two nations' diplomatic battle over Japan's detainment of a Chinese fishing captain. "We believe if our employees knew about the area being off-limits, or the regulation that prohibited cameras in the location, they would not have taken such action," the company's executive director said at a news conference Friday in Tokyo. The four Japanese nationals were sent to China for a Japanese government project to reclaim World War II chemical weapons left by Japan's Imperial Army, their company said Friday. A Chinese national also is missing and presumed arrested with his Japanese co-workers on Wednesday in in northern Hebei Province.
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One of the four sent their company a text message that said, "Help me"
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The four are being investigated for entering a military zone without authorization
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They also may have videotaped military targets, Chinese state media says
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Beijing has made escalating diplomatic threats against Tokyo over a separate arrest
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Japan's foreign ministry officials said Friday that China had informed them of the arrests, but they had no information about any charges or why the @placeholder nationals had been detained.
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He has been accused of obstructing @placeholder public officers while they performed duties.
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The captain and his crew were arrested illegally, according to @placeholder.
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In response to the fishing crew's arrests, @placeholder has halted talks with Japan about increasing civil flights and expanding aviation rights between the two countries.
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@placeholder has urged Beijing to proceed calmly and cautiously to avoid further deterioration of the situation.