The Chinese man reportedly claimed he wished to defect, but Taiwanese officials suspect he could be probing defences.

The man was arrested by Taiwan’s coastguard after crossing the 160km (100-mile-wide) Taiwan Strait that separates the island state from China, and entering the mouth of the Tamsui River in his small craft.

He told authorities that he wanted to defect. But officials say he may have been testing Taiwan’s response to his approach.

The 60-year-old, who Taiwan said was an officer in China’s People’s Liberation Army Navy surnamed Ruan, is now under investigation and being questioned.

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

Taiwan Defense 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.

The phrase is used to refer to irregular tactics to evaluate an area without engaging in open combat and has included China sending boats and surveillance balloons over the island.

Taiwan declared independence from China during a civil war in 1949. However, Beijing insists that the island is part of its territory.

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

According to Kuan of the OAC, there have been 18 similar cases to the one on Sunday over the past year or so, mostly involving islands under the control of Taiwan that are close to the Chinese coast.

“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.

She said Taiwan plans to step up its security measures and that personnel managing radar operations and monitoring stations who failed to detect and stop the boat sooner will be held responsible for the incident.

In March, two Taiwanese fishermen strayed into Chinese waters near the Kinmen islands, one of the territories close to the Chinese coast. One, a Taiwanese military officer, remains in detention in China, while the other was released soon after.

The latest incident comes amid rising tensions between China and Taiwan, and amid concerns China could resort to military action in staking its claim over the island.