"It is not ruled out that the small boat stowaway attempt could be a grey zone intrusion linked to the military exercise," the coast guard said in a statement, referring to tactics which fall short of a direct act of war.

The man used a rubber boat to make the "illegal crossing" to the tiny island of Menghu on Monday, the coast guard said in a statement.

Menghu, which is part of the Taipei-administered Kinmen islands, is about six kilometres (four miles) from mainland China's port city of Xiamen in Fujian province.

Video released by Taiwan's coast guard showed its personnel tying the man's wrists with plastic zip ties and escorting him onto a vessel.

The man and his boat were taken to Kinmen for investigation.

The coast guard said it had "heightened its vigilance" and was constantly monitoring "security hotspots" and carrying out "real-time tracking of maritime targets".