Self-ruled, democratic Taiwan lives under constant threat of invasion by authoritarian China, which views the island as part of its territory to be retaken one day -- by force if necessary.

Under a law passed by Congress, the United States is required to sell Taiwan military supplies to ensure its self-defence against Beijing's vastly larger armed forces.

US President Joe Biden, meanwhile, has come under bipartisan pressure to deepen ties with Taiwan as Washington vies for influence in the Asia-Pacific region to counter China.

Washington announced Wednesday it had approved the sale of spare and repair parts for ships and ship systems, as well as "logistical technical assistance", all of which would enhance Taiwan's "interoperability with the United States and other allies".

"The proposed sale will contribute to the sustainment of the recipient's surface vessel fleet, enhancing its ability to meet current and future threats," the Pentagon's Defense Security Cooperation Agency said in a statement.

It would be the fourth arms sale to Taiwan under Biden, and the third this year.

- 'Practical needs' -

Taipei on Thursday said the deal was expected to take effect in one month.

The sale would help Taiwan's ships "maintain proper equipment... and meet the practical needs of combat readiness tasks in light of recent frequent activities by Chinese aircraft and warships around our sea and air", Taipei's defence ministry said in a statement.

China has ramped up its sabre-rattling since Taiwanese President Tsai Ing-wen took office in 2016. She rejects its stance that the island is part of Chinese territory.

Its foreign ministry on Thursday said Beijing "firmly opposes and strongly condemns" the arms sale.

"China will continue to take resolute and strong measures to defend its own sovereignty and security interests," spokesman Zhao Lijian told reporters at a routine briefing.

Taiwan recorded 969 incursions by Chinese warplanes into its air defence zone in 2021, according to a database compiled by AFP -- more than double the roughly 380 in 2020.

The figure this year has exceeded 470.

Last week China said it firmly opposed US-Taiwan trade talks, after Taipei and Washington announced the launch of a new initiative to deepen economic ties.

In late May on a visit to Japan, Biden appeared to break with decades of US policy when, in response to a question, he said Washington would defend Taiwan militarily if it were attacked by China.

The White House has since insisted the US policy of "strategic ambiguity" over its response to such an attack has not changed.