Taiwan suggests China's trawler seizure may be psychological warfare

China's seizure of a Taiwanese trawler operating close to the Chinese coast for illegal fishing might be an act of psychological warfare by Beijing designed to further pressure Taipei, a top Taiwan security official said on Thursday.

Chinese officials boarded and then took the boat back to a port in China late on Tuesday. It was fishing for squid in Chinese waters but near the Taiwan-controlled Kinmen islands. China says it violated a summer ban on fishing and carried out illegal trawling operations.

The incident happened at a time of growing tensions across the Taiwan Strait. China views democratically governed Taiwan as its own territory and has ramped up pressure on Taipei since May when President Lai Ching-te took office, a man Beijing accuses of being a "separatist".

Speaking to reporters at parliament, Taiwan National Security Bureau Director-General Tsai Ming-yen said it was unusual for Chinese officials to board and then detain a Taiwanese trawler, and noted China's explanation that it was illegally fishing.

But China may also have done this to demonstrate its jurisdiction over the strait and put pressure on Taiwan's government, he added.

"We must continue to analyse whether this is a cognitive warfare operation, and will fully assess what the motivations are for the Chinese communists," Tsai said.

China has seized Taiwanese trawlers before for illegal fishing but released them after a fine was paid, Taiwan's coast guard said.

Late Wednesday, China's Taiwan Affairs Office said the seizure of the boat was an act of "normal law enforcement" to protect fishery resources and the environment given the illegal fishing it was carrying out.

"The relevant departments will deal with it in accordance with the law and regulations," it said in a statement.

Three of the five fishermen on board are Indonesian nationals, according to Taiwan's coast guard.

China says it has sovereignty over the strait. Both Taiwan and the United States say it is an international waterway.

Taiwan has complained for four years of stepped-up Chinese "grey zone" activities attempting to wear down Taiwan with offensive actions that stop short of full-blown conflict, like flying balloons over Taiwan.