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-run media said.
The four work for mid-size Japanese construction company named Fujita Corp., according to the business. They were sent to China for a Japanese government project to reclaim World War II chemical weapons left in China by Japan's Imperial Army, a company official said Friday.
The company last heard from the four employees on Tuesday, the official said. 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 four had been detained.
Fujita is a Tokyo-based general contractor that Goldman Sachs Group acquired in April 2009.
The arrests come as China and Japan have been involved in a diplomatic battle over Japan's detainment of a Chinese fishing captain. He was arrested on September 8 off the disputed Diaoyu Islands, in the East China Sea. Japanese authorities have extended his detainment until at least September 29, while they decide how to handle his case. The captain has been accused of obstructing Japanese public officers while they performed duties.
Beijing has made escalating diplomatic threats against Tokyo as it has demanded his release.
China has halted talks with Japan about increasing civil flights and expanding aviation rights between the two countries, and officials and nationals on both sides have cancelled trips to each other's nations.
Japan has already freed the captain's 14 crew members, who have flown back to China, and returned their fishing boat.
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.
China's Foreign Ministry has said that Japan has "seriously damaged Sino-Japan bilateral relations."
Tokyo has urged Beijing to proceed calmly and cautiously to avoid further deterioration of the situation.
CNN's Kyung Lah and Yoko Wakatsuki contributed to this report.