Japan's coast guard prevented ethnic Chinese activists on Friday from landing on Japanese-held islands also claimed by China and Taiwan, a fresh reminder of territorial disputes that dog Tokyo's ties with its neighbours.
The boat carrying the activists turned away from the uninhabited East China Sea islets, known as the Senkakus in Japan and the Diaoyutai in China, and left Japanese waters after Japanese patrol ships warned it not to go any closer and sprayed it with water.
Twenty Japanese coast guard vessels blocked its way as three helicopters hovered around, the group that sent the boat said, adding that it was now heading back to Hong Kong.
An ageing fishing trawler had set sail from Hong Kong on Sunday carrying some 20 protesters - from Hong Kong, the US and Canada and elsewhere - who claim China's sovereignty over the islands.
"We have said that Japanese authorities will deal sternly and take steps to expel them if they enter and land on Japanese territory," Chief Cabinet Secretary Yasuhisa Shiozaki said.
The Japanese coast guard declined to disclose the number of ships involved or other details of the operation, but the activists' group said the boat had collided with a coast guard ship and had suffered damages to its front and side.
"The central (Beijing) government should lodge a serious complaint with Japan because they endangered our crew members, they also failed to respect our sovereignty over the islands," said Ku Kwai-yiu, a member of the group that organised the protest mission.
China, Taiwan and Japan all claim the islands, 170 km (100 miles) northeast of Taiwan and 410 km (250 miles) west of Japan's Okinawa island. Some analysts say they sit above underwater deposits of oil and gas and around rich fishing grounds.
"Historically, it's clear that they are Japanese territory," Japanese Foreign Minister Taro Aso said earlier on Friday.
The incident served as a reminder to Tokyo and Beijing of the thorny territorial feud just as the countries see a thaw in their ties brought about by Japanese Prime Minister Shinzo Abe's visit to China this month.
In September 1996, Chinese activist David Chan died after jumping into rough seas from a chartered Hong Kong vessel to protest against Japan's claim to the islands, and in March 2004, Japanese police arrested seven Chinese activists who landed on one of the islands.
The islands are also near gas fields disputed between Japan and China, another of the several issues that have dragged down ties between the Asian neighbours.
The Chinese Foreign Ministry has not commented on the latest incident, but its spokesman said on Sunday that Beijing has "strongly demanded" Japan react to the protest calmly and not jeopardise the safety of the activists and their vessel.
Abe's meeting with Chinese President Hu Jintao in Beijing was the first between the two countries' leaders since April 2005.
Hu had shunned meetings with Abe's predecessor, Junichiro Koizumi, due to his annual visits to Tokyo's Yasukuni Shrine for the war dead, seen by Beijing as a symbol of Japan's wartime militarism.