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 . They work for Fujita Corp. , a mid-size Japanese construction company that Goldman Sachs Group acquired in April 2009 . Fujita last heard from its employees on Tuesday , the company Executive Director Tatsuro Tsuchiya said Friday . The Fujita office in China received a text message from one of the employees that said , `` Help me , '' in Chinese . The company has n't heard from them since . Fujita 's representative in China is on the way to Hebei Province and has reached out to Japan 's Foreign Ministry for help , Tsuchiya said . `` If they are really being detained , we hope nothing but their early release , '' he said . `` We have very limited information . We strongly hope this situation is solved quickly , '' he added . 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 Japanese nationals had been detained . `` Currently , the case is being investigated , '' is all that Chinese security authorities in Shijiazhuang said in a statement , the state-run China Daily reported . But China has had plenty to say about Japan detaining the Chinese fishing captain . Beijing has made escalating diplomatic threats against Tokyo as it has demanded his release . On Friday , Japan said it will release the captain , but did not offer a time frame . The captain was arrested on September 8 off the disputed Diaoyu Islands , in the East China Sea . He has been accused of obstructing Japanese public officers while they performed duties . Japan has already freed the captain 's 14 crew members , who have flown back to China , and returned their fishing boat . The captain and his crew were arrested illegally , according to China . Beijing says the Diaoyu Islands and most of the South China Sea belong to China , disputing neighboring countries ' claims . In Japan , the islands are known as the Senkaku . The clash over territorial waters and islands -- and the natural resources that go with them -- is a flashpoint in the Asia-Pacific region . In response to the fishing crew 's arrests , China has halted talks with Japan about increasing civil flights and expanding aviation rights between the two countries . Officials and nationals on both sides also have canceled trips to each other 's nations . China 's Foreign Ministry has said that Japan has `` seriously damaged Sino-Japan bilateral relations '' with the fishing crew 's arrests . Tokyo has urged Beijing to proceed calmly and cautiously to avoid further deterioration of the situation . CNN 's Yoko Wakatsuki contributed to this report .
One of the four sent their company a text message that said , `` Help me '' The four are being investigated for entering a military zone without authorization . They also may have videotaped military targets , Chinese state media says . Beijing has made escalating diplomatic threats against Tokyo over a separate arrest .