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

Beijing has significantly increased incursions into Taiwan's air defence identification zone in recent months, with the final quarter of 2021 seeing a massive spike of incursions.

On Tuesday, Taipei expressed "gratitude" to the United States for approving a $100 million arms sale, which will maintain its Patriot missile air defence system.

"This is the second arms sale to Taiwan since President Joe Biden took office and the first time this year," Taiwan's presidential spokesman Xavier Chang said in a statement.

"It ... reflects the rock-solid partnership between Taiwan and the US."

Taiwan's defence ministry said the deal is expected to take effect sometime in March.

According to a statement from US Defence Security Cooperation Agency, the deal covers engineering support and maintenance of air defence systems and will "ensure readiness for air operations".

"The recipient will use this capability as a deterrent to regional threats and to strengthen homeland defense," said the US agency.

- Incursions -

The first major arms sale to Taipei under Biden was last August, which saw the approval of 40 155mm M109A6 medium self-propelled howitzer artillery systems.

Beijing has become increasingly bellicose under Chinese President Xi Jinping, ramping up military, diplomatic and economic pressure on Taiwan.

Last year, Taiwan recorded 969 incursions by Chinese warplanes, according to a database compiled by AFP -- more than double the roughly 380 carried out in 2020.

The new year appears to be following the same trend: the defence ministry recorded 52 warplanes entering the zone over two days in January, a large uptick over a short period.

The Patriot is a highly mobile surface-to-air missile system that would be a crucial defence against Chinese warplanes.

Washington diplomatically recognises Beijing over Taipei but it is also bound by Congress to sell Taiwan weapons to defend itself.

Former US president Donald Trump ramped up big-ticket arms sales to Taiwan during his time in office as he feuded with Beijing on a host of issues from trade to national security.

Sales included drones, missile systems and new-generation fighter jets.