Taiwan's coast guard picked up the man on Sunday after his vessel collided with other boats on the Tamsui River, which flows from the capital Taipei to the island's northern coast.

The man, 60, told coast guard officers at the scene that he wanted to "defect", according to the semi-official Central News Agency.

Taiwan's Ocean Affairs Minister Kuan Bi-ling said the man had served as a captain in the Chinese navy, adding he was one of 18 purported defectors seen over the past year or so, all of whom claimed they admired self-ruled "Taiwan's democratic way of life and came for freedom".

"We often encounter this kind of rhetoric... we do not believe it and we will investigate it from all aspects," she told reporters.

"What's different this time is... the man is more refined and better dressed, and his experience is more special," Kuan said, alluding to his naval service.

"We do not rule out that this is a test," she continued, adding she was "very sorry" for the breach.

China claims self-ruled democratic Taiwan as part of its territory, and it has ramped up military and political pressure in recent years, including with war games last month that it called a test of its ability to seize the island.

The Taiwan Strait -- a 180-kilometre (110-mile) channel separating China from the island -- is one of the world's most heavily policed waterways.

Taiwanese Premier Cho Jung-tai told reporters Tuesday that "all national security units and teams are paying close attention, and investigations have been launched".

"National security cannot be neglected for a minute," he said, adding that relevant units had been instructed to "immediately strengthen protective measures".

Taiwan is routinely on guard for spies from China.

The coast guard said it first began monitoring the boat on Sunday when it was spotted around 9 am (0100 GMT) about six nautical miles (11 kilometres) off the coast of Tamsui, a district in the suburbs of Taipei.

Speaking Tuesday during a press conference, deputy coast guard chief Hsieh Ching-chin said personnel would be held accountable for failing to intercept the boat due to "misjudgement and negligence".

He explained that a radar operator had incorrectly believed the vessel to be a local fishing boat, while the units notified to identify it did not intercept or board it for inspection.

In 2021, a Chinese man was arrested in central Taichung city after he managed to cross the Taiwan Strait in a rubber dinghy.

He said he had come to Taiwan to seek "freedom and democracy", according to police.

He was deported to China in 2022, according to local media.