The group of 14 activists left for the disputed islands -- known as Senkaku in Japanese and Diaoyu in Chinese -- on a Chinese-flagged fishing boat from Hong Kong.

The activists, who belong to a group called the Action Committee for Defending the Diaoyu Islands, will be joined at sea by two other vessels on Tuesday -- from Taiwan and Xiamen city in southern China.

"We are protesting Japanese lawmakers' plan to land on the islands on August 19," said Chan Miu-tak, the group's chairman.

"We want to reach the islands before the Japanese lawmakers and place the Chinese flag there to show our determination to safeguard China's sovereignty over the islands," he told AFP.

A group of lawmakers from Japan have said they plan to visit the disputed islands next weekend.

Chan said the vessels are expected to reach the disputed islands on Wednesday if they are not turned back by authorities.

The pro-China group has made repeated attempts to reach the islands, but apart from one successful foray in 1996 they have been blocked by Japanese patrol vessels.

China and Japan have a lengthy dispute over the uninhabited but strategically coveted island chain, which is also claimed by Taiwan.

The move came after South Korea's President Lee Myung-Bak paid a surprise visit on Friday to islands at the centre of a decades-old territorial row with Japan, which recalled its ambassador from Seoul in protest.

The potentially resource-rich sea, home to important trade routes, is an increasingly dangerous flashpoint and there have been a string of recent diplomatic rows between countries with overlapping territorial claims.

Taiwan's drill will take place on Taiping Island in the Spratlys, a sprawling group of islands claimed in whole or part by Taiwan, China, Vietnam, Malaysia, the Philippines, and Brunei, the coastguard said.

The coastguard said it would take place in September but did not give an exact date.

Vietnam already voiced anger after Taiwan last week moved new weaponry to the island in preparation for the drill, sparking a rebuke from Taipei.

"Taiping Island is one of the islands in the area that has long been governed by" Taiwan, the foreign ministry said in a statement. "Its sovereignty is indisputable."

The drill on the island, the biggest in the Spratlys, involves newly arrived 40mm artillery and 120mm mortars, the Taipei-based United Evening News said, which also reported that several lawmakers will watch the drill.

The range of the 120mm mortars is 6.1 kilometres (3.8 miles), compared with 4.1 kilometres (2.5 miles) for the mortars currently in use by Taiwanese coastguards on Taiping Island, the paper said.

All claimants except Brunei have troops based on the archipelago of more than 100 islets, reefs and atolls, which sprawl across a vast area but have a total land mass of less than five square kilometres (two square miles).

South China Sea tensions escalated last month when China announced a new city and military garrison in the dipusted Paracel Islands, prompting Washington to voice concern.