Beijing, which claims self-ruled Taiwan as part of its territory, neither confirmed nor denied the manoeuvres.

Taiwan's defence ministry and other security agencies were monitoring China's activities and had a "complete grasp of the situation", presidential office spokeswoman Karen Kuo told reporters.

She did not say how many Chinese ships were involved in the deployment, but a security source told AFP the number was "significant". The source spoke on condition of anonymity because they were not authorised to speak to the media.

The operations were not limited to the Taiwan Strait, but extended from the southern Yellow Sea, to the East China Sea near the disputed Diaoyu Islands and on into the South China Sea and even the Western Pacific, Kuo said.

"This indeed poses a threat and impact on the Indo-Pacific and the entire region," she said.

Taiwan urged China to "exercise restraint", Kuo said, adding: "We are also confident that we can handle this matter well."

Neither China's armed forces nor state media have announced any increased military activity in the region where Taiwan said Chinese ships had been detected.

Beijing's foreign ministry did not answer directly when asked if the military was amassing a large number of naval vessels in those waters.

"I want to emphasise that China has consistently followed a defensive policy," spokesman Lin Jian told a regular press briefing.

"The Chinese navy and coast guard strictly operate in relevant waters according to Chinese domestic law and international law," Lin said.

He urged "relevant parties" not to "overreact or... engage in groundless hype".

China has refused to rule out using force to take Taiwan, and also claims contentious sovereignty over nearly all of the South China Sea.

Taiwan's intelligence chief Tsai Ming-yen said Wednesday that October to December was the "peak season" for China's "annual evaluation exercises".

There was a possibility that China's ruling Communist Party could turn seemingly routine military activities into drills targeting Taiwan, Tsai warned.