China says Taiwan is part of its territory and claims jurisdiction over the Taiwan Strait, a major international shipping route.

The United States, Britain and other countries view the strait as international waters open to all vessels.

Eight countries, including the United States, Japan, Australia and France, have sent naval ships through the narrow stretch of water so far this year, National Security Bureau director-general Tsai Ming-yen said.

China shadows "every vessel", Tsai told lawmakers in parliament.

"It will deploy corresponding naval assets to conduct necessary monitoring, and at times... it will also mobilise air forces to carry out simulated attacks to signal its military presence and claim of dominance in the Strait."

Taiwan exchanges intelligence with its "international allies", including on Chinese military activity during these transits, Tsai said.

Tensions across the strait have worsened in recent years as China has increased military pressure on Taiwan and Taipei has ramped up its defence spending.

Taiwanese President Lai Ching-te last week unveiled plans for $40 billion in extra defence spending over the next eight years, with a focus on developing a multi-layered air defence system.