"China's foreign ministry is now urgently contacting the Japanese side to lodge representations over this matter," the official Xinhua news agency said.

In a separate report, Xinhua said the ministry would "lodge a complaint" with Japan but gave no further details of what action Beijing might undertake.

Japanese police held nine more people late Wednesday after authorities arrested five activists on the disputed archipelago known as Diaoyu in China and Senkaku in Japan.

The pro-China activists from Hong Kong and Macau set sail to the disputed islands on a Chinese-flagged fishing boat from Hong Kong on Sunday.

The activists, who belong to the Action Committee for Defending the Diaoyu Islands, had said the move was aimed at countering a plan by a group of Japanese lawmakers to visit the disputed islands this weekend.

Earlier on Wednesday, China's foreign ministry urged Japan to protect the safety and property of activists headed to the islands, which are claimed by both nations.

"China is paying close attention to the developments of relevant issues and it has expressed its serious concerns over the issues to the Japanese side," foreign ministry spokesman Qin Gang said in a statement.

Japanese Prime Minister Yoshihiko Noda told reporters that he will "deal with the incident strictly in line with the law".