While Washington is traditionally Taiwan's biggest arms supplier, remarks by US President Donald Trump raised doubts about his willingness to defend the democratic island.

Taiwan has ramped up its defence spending in the past decade as China has intensified military pressure, but Trump's administration has pushed the island to do more to protect itself.

The arms sale announced on Thursday, which still needs US Congressional approval, would be the second since Trump returned to office in January, after the $330 million sale of parts and components in November.

The latest, much bigger cache features HIMARS rocket systems, howitzers, anti-tank missiles, drones and other equipment, according to Taipei's foreign ministry.

"This is the second arms sale to Taiwan announced during the Trump administration's second term, once again demonstrating the US's firm commitment to Taiwan's security," Taipei's foreign ministry said.

The potential size of the sale rivals the $18 billion authorised under former US president George W. Bush in 2001, although that was ultimately downsized after commercial negotiations.

Bush ended up selling $15.6 billion-worth of weapons to Taiwan over his eight years in office.

During Trump's first term, the United States approved $10 billion in arms sales to Taiwan, including $8 billion for fighter jets.

The latest package is expected to soon receive a Congressional rubber stamp, given the cross-party consensus on Taiwan's defence.

Taiwan maintains its own defence industry but the island would be massively outgunned in a conflict with China, and so remains heavily reliant on US arms.

The latest arms sale shows Washington has continued to assist Taipei in "rapidly building robust deterrence capabilities", Taiwan's defence ministry said in a statement.

Taiwanese President Lai Ching-te's government has vowed to ramp up defence spending to more than three percent of the GDP next year and five percent by 2030, following US pressure.

It is also plans to seek up to NT$1 trillion in special funding to upgrade the island's air defence systems and increase capacity to produce and store ammunition.

The defence spending proposals need backing from the island's opposition-controlled parliament before they can take effect.

China deploys military aircraft and warships around Taiwan on a near-daily basis, which analysts describe as "grey-zone" operations -- coercive tactics that fall short of an act of war.

Taipei's defence ministry said 40 Chinese military aircraft, including fighters, choppers and drones, as well as eight naval vessels, were detected around Taiwan in a 24-hour period ending early Thursday.

On Tuesday, Beijing's third and newest aircraft carrier, the Fujian, sailed through the Taiwan Strait, according to Taipei.