BEIJING (Reuters) - China's government said on Wednesday that a Chinese man arrested near Taipei after crossing the Taiwan Strait on a speedboat was acting on his own and that Taiwan should not be so suspicious, in a case that has alarmed Taiwanese security officials.

Taiwan's coast guard arrested the man on Sunday in the coastal neighbourhood of Tamsui after his boat entered a river that leads into Taipei amid ongoing tensions between Taiwan and China.

Taiwan says the man is a former Chinese navy captain who could have been probing the island's defences.

Speaking at a regular news briefing in Beijing, Chen Binhua, spokesperson for China's Taiwan Affairs Office, said what the man had done was "purely his personal action".

"There is no need for the Democratic Progressive Party authorities to see soldiers hiding behind every tree and bush, carrying out political manipulation while pretending to act earnestly," he said, referring to Taiwan's ruling party.

The man will be punished once he returns to China, Chen added, without giving details.

Taiwan, which did not detect his boat before it got close to the coast, has not indicated when or if it will send him back, saying the legal authorities are still investigating.

Taiwan National Security Bureau Director-General Tsai Ming-yen, speaking at parliament in Taipei on Wednesday, said they were not excluding any possibilities for the man's motivation, including whether China's government was involved.

The man's apparent military background made the case rather unusual, Tsai added.

"There are many areas of doubt that still need to be further clarified," he said.

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 flying surveillance balloons over the island. Tsai said the speedboat incident might be just such a tactic.

Taiwan's coast guard said that the man crossed the strait from China's Fujian province, which sits opposite Taiwan, and that it does not believe he came to Taiwan seeking political asylum.

(Reporting by Liz Lee; Writing and additional reporting by Ben Blanchard in Taipei. Editing by Gerry Doyle)