China yesterday called on Indonesia to release 75 captured Chinese fishermen and free their impounded vessels "as soon as possible", just six days ahead of a scheduled visit from Indonesia's foreign minister.

Beijing made the demand after Jakarta seized eight vessels that originated from the Guangxi Zhuang autonomous region on Saturday.

The vessels were all in a "traditional fishing area" in the waters around China's Nansha Islands, Foreign Ministry spokesman Qin Gang said.

"China is gravely concerned about this incident," Qin said, while expressing "strong dissatisfaction".

According to the Chinese embassy in Jakarta, the fishermen were seized by the Pontianak Fishery and Ocean Resources Monitoring Agency.

At press time, they were still in detention and the embassy was seeking talks with Indonesian authorities.

"The embassy is on the way to meet the Chinese fishermen in Pontianak in the West Kalimantan," the embassy's press officer told China Daily.

The Jakarta Post on Tuesday quoted the head of the Pontianak Fishery and Ocean Resources Monitoring Agency, Bambang Nugroho, as saying his office was responsible for stopping the eight vessels and their crew of 77. A source from the fishery administration bureau under China's Ministry of Agriculture confirmed that the actual number of people captured was 75 - all from Guangxi.

It was "the biggest case in recent years", the source said, noting that Indonesia captured four Chinese fishermen in March 2008 and freed them one year later.

Antara, Jakarta's state news agency, said the Chinese were fishing in Indonesia's Exclusive Economic Zone, near Natuna.

Wang Hanling, an expert in maritime affairs and international law at the Chinese Academy of Social Sciences, said the fishermen were within a traditional Chinese fishing area.

"Indonesia's unilateral exercising of administration over the disputed waters is illegal and invalid as it violates international law," Wang said.

"It is unreasonable and unacceptable that Indonesia arrested Chinese fishermen within the maritime area historically under China's administration," he said.

According to Wang, the arrest signaled Indonesia's intention to dispute sovereignty of some waters in the South China Sea.

"The ultimate solution to disputes always rests with talks and friendly consultation," Wang said.

Indonesia's Foreign Minister, Hassan Wirajuda, is set to begin a two-day visit to China next Wednesday.