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 hasn'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.