TAIPEI >> Chinese officials boarded and then seized a Taiwanese fishing boat operating near China’s coast close to a Taiwan-controlled island late Tuesday and took it to a Chinese port, Taiwan’s coast guard said, in a further escalation of tensions.

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”.

The squid fishing boat was near the Taiwan-administered Kinmen islands, which sit next to the Chinese cities of Xiamen and Quanzhou, but in Chinese waters on Tuesday night when it was boarded and seized by two Chinese maritime administration boats, Taiwan’s coast guard said.

The Taiwanese boat was operating during China’s no-fishing period, the coast guard said, adding Taiwan will communicate with China and urge them to release the fishermen as soon as possible.

Coast guard officials in southern Fujian seized the Taiwan boat, China’s coast guard said in a statement, as it violated a summer ban on fishing and carried out illegal trawling operations.

The nets used were also far smaller than the minimum size China stipulates, so damaging marine fishery resources and the environment, it added.

During this period, Taiwan vessels tried to “interfere with” China’s normal law enforcement, and the Fujian officials warned and “drove them away” in line with the law, it said.

China’s Taiwan Affairs Office did not immediately respond to a request for comment.

White House spokesperson Karine Jean-Pierre told reporters on Wednesday that Washington was closely monitoring the incident.

Taiwan sent its own coast guard ships to assist and broadcast warnings asking China to release the fishing boat, but China’s ships broadcast back saying not to interfere, Taiwan’s coast guard said.

Taiwan’s ships then backed off to avoid a conflict and the Taiwanese fishing vessel was then taken to a Chinese port, it added, saying three of the five fishermen were Indonesian migrant workers.

Taiwan Coast Guard Administration Deputy Director-General Hsieh Ching-chin told reporters in Taipei that China should explain why it had seized the boat, and pointed out that in previous cases, fishermen had been released after paying fines when operating during China’s no-fishing season.

Taiwanese fishing boats need to raise their alert level and the coast guard will also strengthen its patrols, he added.

“The coast guard also calls on the mainland side not to use political factors to handle this situation,” Hsieh said.

Taiwan’s top China policy-making body the Mainland Affairs Council said it will contact Chinese authorities to seek an explanation of what had happened and “dispel unnecessary speculation”.

Judha Nugraha, director for citizen protection at Indonesia’s foreign ministry, told Reuters the country’s consulate general in Guangzhou will assist the detained Indonesians.

This is not the first time a Taiwan fishing boat has taken by Chinese authorities after operating in that country’s waters, an official said, speaking on condition of anonymity given the sensitivity of the situation.

A Taiwan official, who’s familiar with the island’s security planning, told Reuters they have issued alerts to fishing and transport authorities around Taiwan to pay attention to “possible risks” amid frequent Chinese coast guard activities in the region, including near Japan and the Philippines.

It is not uncommon for Taiwan and China to detain each other’s trespassing fishing boats. So far this year Taiwan has detained five such boats from China, Taiwan coast guard data shows.

Chinese maritime enforcement and coast guard ships have been regularly operating around Kinmen since February after two Chinese fishermen died trying to flee Taiwan’s coast guard.