Beijing, China (CNN) -- China on Thursday released three Japanese nationals after they admitted illegally entering a Chinese military zone and expressed regret, state-run media said.
Another Japanese national, who worked with the three, remained under house arrest for illegally videotaping military targets and an investigation was continuing, the Xinhua News Agency said.
"We will continue our effort to rescue the one left behind," Fujita Corp., the company that employs the four Japanese nationals, said Thursday.
Fujita said it did not know the whereabouts of the three whom China had released.
All four Japanese nationals were arrested in Hebei Province on September 23 as Beijing and Tokyo clashed over the arrest of a Chinese fishing captain by Japan.
The captain was taken into custody in early September off the disputed Diaoyu Islands, in the East China Sea. His arrest touched off a battle that escalated into diplomatic threats by Beijing, the suspension of diplomatic talks and canceled trips between the nations.
Japan has since freed the fishing captain, but the two countries have continued the diplomatic battle. Both nations have demanded compensation over the captain's arrest. China says he was arrested illegally and wants damages for his damaged fishing boat. Japan said he damaged two Japanese patrol boats and has demanded compensation as well.
"We hope Japan can meet China halfway to reduce the negative impact of the incident and prevention of future incidents," Jiang Yu, spokeswoman for China's Ministry of Foreign Affairs, said late Thursday afternoon about relations between the two countries.
China had released no further details about the four Japanese nationals, or a Chinese co-worker who went missing at the same time as the arrests. The Chinese national was presumed arrested as well.
Jiang said only that an investigation is under way.
" I believe the relevant authorities will handle the case according to the law," Jiang said.
All five workers were employed by Fujita Corp., a midsize Japanese construction company. The four Japanese employees were sent to China for a Japanese government project to reclaim World War II chemical weapons left by Japan's Imperial Army, Fujita said.
Goldman Sachs Group acquired Fujita in April 2009.
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 flash point in the Asia-Pacific region.
"Reiterate [that] Diaoyu Islands are inherent territory, and the incident was severe infringement of Chinese sovereignty and human rights," China's spokeswoman Jiang said. "We stand by our recent statement, expounded on the Chinese government's position of territorial integrity and our willingness and goodwill to develop Japan relations."
Japan has recently urged China to not unilaterally develop a gas field in the disputed area of the East China Sea, Japan's Ministry of Foreign Affairs said Thursday.
"Unilateral measures are not in accordance with this [bilateral] agreement and not conducive for the two countries," Deputy Press Secretary Hidenobu Sobashima said.
"We are concerned if there's a move in that direction," he added. "Yes, that message has been conveyed at the ambassador level."
Jiang said in response: "Our position is solemn and earnest: We want no further deterioration of the relations. We hope Japan makes no new obstacles. China has complete sovereignty over the ... gas fields. China's activities there are reasonable and lawful."
CNN's Junko Ogura and Kyung Lah contributed to this report.