BEIJING -- A boat carrying four volunteers for a Chinese activist group who tried to stage a protest on a small group of disputed islands in the East China sea was on its way home Monday after being blocked by Japanese patrol boats.

The four from the activist group Chinese Foundation of Diaoyu Defense, who say the islands are part of China, set off from Xiamen, in southeast Fujian province, and came within 13 kilometers (8 miles) of the islands Sunday evening, according to their spokesman Li Yan.

They were hindered by three Japanese naval ships using water cannon and one plane, he said, as they held up a Chinese flag.

There were no known casualties. The activists will spend a day and night at sea before they reach home, Li said.

Japan seized the islands in 1895 when it colonized Taiwan, which also claims the territory as its own. They are surrounded by rich fishing grounds and are regularly occupied by activists from the parties involved.

They are known as the Diaoyu Islands in China and Senkaku in Japan.

The Japanese coast guard had stopped following the boat Monday, Kyodo news agency reported.

"If Diaoyu islands are not given back to China our activity will not stop," Li told The Associated Press.

He said the timing of the protest trip was due more to the weather than any change in Chinese-Japanese relations.

The Chinese Foundation of Diaoyu Defense group was set up in 2003 and has made several previous protests. According to Hong Kong paper Ta Kung Pao, an attempt in August was abandoned a few days before it was to leave Hong Kong after the government there scrapped the boat's registration.

China remains wary of upsetting relations with Japan, although anti-Japanese sentiment has flared up in China in the past without being calmed by the Chinese government.

Japan lodged a protest with China through diplomatic channels Sunday following the incident, the Japanese Foreign Ministry said in a statement.

Tokyo told Beijing that the islands "are no doubt our territory historically and according to international law," according to the ministry. Tokyo also said, "It is extremely regrettable something like this happened and we strongly protest," according to the statement.