TAIPEI >> A Chinese man arrested after his speedboat illegally entered a Taipei harbor is a former navy captain who could have been probing the island’s defenses, senior Taiwanese officials said on Tuesday.

Taiwan’s coast guard arrested the man on Sunday at the coastal neighborhood of Tamsui after his boat entered a river that leads into Taipei, an incident that happened amid ongoing tensions between Taiwan and China.

China views the democratically governed island as its own territory, a claim Taipei rejects.

Kuan Bi-ling, head of Taiwan’s Ocean Affairs Council, which runs the coast guard, told reporters at parliament that the man was “quite refined and well presented” and had previously served as a Chinese navy captain.

Over the past year or so there have been 18 similar cases, mostly involving Taiwan-controlled islands that sit next to the Chinese coast, Kuan said.

“Looking at the accumulated cases in the past, we can’t rule out that this is a test,” she said, referring to Taiwan’s abilities to spot such vessels.

Neither China’s Taiwan Affairs Office nor its defense ministry immediately responded to requests for comment.

Taiwan Defence Minister Wellington Koo, also speaking to reporters at parliament, said the boat incident could be another example of China’s “grey zone” tactics against the island.

Taiwan has complained in recent years that China has been using so-called grey zone warfare designed to exhaust a foe by irregular tactics without resorting to open combat, such as floating surveillance balloons over the island.

“These grey zone tactics have always existed,” Koo said. “We must always maintain our vigilance and cannot rule out the possibility of taking countermeasures.”

In March, two Taiwanese fishermen strayed into Chinese waters near the Taiwan-controlled Kinmen islands, which sit next to China’s coast. One, a Taiwanese military officer, remains in detention in China, while the other was released soon after.