Experts Monday called on Japan to show restraint over a Chinese activist group's recent assertion of China's sovereignty over the islands, one day after a Hong Kong activist group set sail for the Diaoyu Islands in the East China Sea.

The ship, belonging to the Action Committee for Defending the Diaoyu Islands, set sail on Sunday noon from Hong Kong. A total of 15 activists were on board, including four crew members and 11 activists from Hong Kong, Macao and the Chinese mainland.

They planned to meet with the vessels of their Taiwanese counterparts in the Taiwan Straits Tuesday and reach the Diaoyu Islands together on August 15, the memorial day of victory over Japan during World War II, Li Yiqiang, a mainland activist with the group, told the Global Times Monday.

This is the first time in five years the activists have made a voyage to the Diaoyu Isands. Their earlier attempts were stopped by the Hong Kong government.

They brought along the national flags and regional flag of Hong Kong and sang the national anthem on the boat, Li said.

The Ministry of Foreign Affairs made no comment when contacted by the Global Times.

Sankei Shimbun reported Monday that the Japanese Chief of Staff had ordered the Self-Defense Forces to make a battle plan in response to the "provocation" of the Chinese vessels heading for the Diaoyu Islands.

The Embassy of Japan in China told the Global Times that there was no such plan.

The activists said they had no fears for their safety. "I don't think the Chinese fishery administration boats will turn a blind eye to our patriotism," Li said.

China has dispatched fishery administration boats to patrol around waters off the Diaoyu Islands in a bid to assert Chinese sovereignty since tensions near the islands escalated this July.

The activists' voyage was a response to some 50 Japanese lawmakers' plan to set foot on the Diaoyu Islands on August 19.

Reports said the Japanese lawmakers were scheduled to hold a vigil for the victims of a refugee shipwreck during wartime on the islands.

However, the Japanese government has turned down the plan for fear of causing an "unexpected incident" between China and Japan, and having a negative impact on the Japanese government's plan to "nationalize" the Diaoyu Islands, according to Kyodo News.

"Japan's constant aggressive activities have provoked Chinese people's actions on asserting sovereignty over the Diaoyu Islands, and the Japanese government should stay restrained on China's grass-roots activities of defending its territorial sovereignty," Geng Xin, deputy director of the Tokyo-based Japan-China Communication Institute, told the Global Times.

According to Kyodo News, Japan is also considering nominating a new ambassador to China in the Autumn to replace Uichiro Niwa, the current ambassador who reportedly leaned toward Beijing on the Diaoyu issue.

Du Liya contributed to this story