TAIPEI, Taiwan — Taiwan said Wednesday that China warned Taiwan’s coast guard against interfering in the detention of a Taiwanese fishing boat, in what is seen as Beijing’s latest attempt to encroach on Taiwanese territory.

The incident comes amid heightened political tensions following the election of President William Lai Ching-te, whose party rejects unification with the mainland, and an apparent threat by Beijing to execute supporters of Taiwanese independence.

Taiwan’s coast guard repeated its call for the release of the boat and its crew members who were taken from waters off the Taiwanese-controlled island of Kinmen just off the Chinese coast on Tuesday night. That call is complicated by China’s refusal to communicate with Taiwan’s government.

A spokesperson for Taiwan’s coast guard, Hsieh Ching-chin, said the boat was not in Chinese waters when it was boarded by Chinese agents and steered to a port in the Chinese province of Fujian.

“First, we call on the (Chinese side) to provide an explanation, and second to release the boat and its crew,” Hsieh said.

The Dajinman 88 was intercepted by two Chinese vessels, and Taiwan dispatched three vessels to help but the one that got close to the fishing boat was blocked by three Chinese boats and told not to interfere, the coast guard’s initial statement said.

Hsieh said four other Chinese boats joined in the operation, a sign of the massive expansion in recent years of China’s navy, coast guard and maritime militia.

The pursuit was called off to avoid escalating the conflict, Hsieh said.

The boat had a captain and five other crew members, according to Taiwan’s official Central News Agency. The crew are Taiwanese and Indonesian.

The vessel was just over 20 kilometers (12 miles) from Jinjiang in mainland China when it was boarded, Taiwanese authorities said.

China claims self-governing Taiwan as its territory and says the island must come under its control.

Fishermen from both Taiwan and China regularly sail the stretch of water near Kinmen, and tensions have risen as the number of Chinese vessels has increased.

In February, two Chinese fishermen drowned while being chased by Taiwan’s coast guard off the coast of Kinmen, prompting Beijing to step up patrols.

China has been increasing military action around Taiwan, as well as the island groups of Kinmen and Matsu, which lie within sight of the Chinese coast. More worrying is China’s daily dispatch of warplanes and navy ships around the island and military exercises, seen as rehearsals for a potential blockade or invasion.

Taiwan’s Defense Ministry said 20 Chinese People’s Liberation Army Air Force planes crossed the median line of the Taiwan Strait between Tuesday and Wednesday morning.

Last month, China passed new injunctions threatening to hunt down and potentially execute “die-hard Taiwan independance separatists.” In response, Taiwan warned its citizens to avoid visiting the mainland and the semi-autonomous Chinese cities of Hong Kong and Macao.

On Tuesday, a spokesman for the Chinese Cabinet’s Taiwan Affairs Office said the threat would affect only a hard-core minority of Taiwanese, and accused the ruling Democratic Progressive Party of “willfully misinterpreting” the action in an attempt to spread fear.

Taiwanese citizens overwhelmingly favor the island’s current status as de-facto independent, even while it faces military threats and diplomatic isolation imposed by Beijing.

A former Japanese colony, Taiwan rejoined China after World War II, but split away in 1949 as the Nationalists under Chiang Kai-shek were swept from the mainland by Ma Zedong’s Communists. No peace treaty has ever been signed, even as ties including direct flights between the sides have burgeoned.

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Associated Press writers Dake Kang and Didi Tang contributed to this report.

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