China has warned Japan not to ''endanger safety'' of its citizens as a group of Hong Kong activists approached disputed islands by boat.
The activists are due to reach the islands, known as Senkaku in Japan and Diaoyu in China, later in the day, Kyodo news agency said.
The boat is being followed by Japanese vessels, the activists said.
Tensions between the two countries have been rumbling in recent months over the islands in the East China Sea.
The islands, which are also claimed by Taiwan, are controlled by Japan and form part of Okinawa prefecture.
In April, a fresh row ensued after Tokyo Governor Shintaro Ishihara said he would use public money to buy the islands from the current private owner.
China is ''paying close attention'' to the developments, foreign ministry spokesman Qin Gang said in a statement, state-run Xinhua news agency reported.
The latest Hong Kong media reports citing a spokesman for the activists said the fishing boat was about 30 nautical miles from the islands.
The Japanese coast guard has increased security in the area, said an AFP news report.
''We are on high alert so as to prevent them from entering Japanese territorial waters or landing on the islands, while exercising caution to avoid a possible collision," a local official of the coast guard told AFP.
The group of pro-China activists set sail for the islands on Sunday. They were aiming to reach them on the 67th anniversary of Japan's surrender to the Allied forces, ending WWII.
Reports said protesters had also gathered at the Japanese embassy in Beijing and a Japanese government office in Taipei.
The largely uninhabited crop of islands are close to strategically important shipping lanes, offer rich fishing grounds and are thought to contain oil deposits.