Taiwan has spent many billions of dollars upgrading its military in the past decade, but faces growing US pressure to do more to protect itself against China, which claims the island is part of its territory and has not ruled out using force to annex it.

Taiwan's President Lai Ching-te has proposed $40 billion in extra defense spending over eight years, but the plan has been blocked by the opposition-controlled parliament 10 times since early December.

A letter dated February 12 and signed by 37 Republican and Democrat members of the Senate and House said the United States and Taiwan "must do more to deter PRC aggression," referring to the People's Republic of China.

China's President "Xi Jinping is focusing every element of the PRC's national power to control Taiwan," the lawmakers said.

"Xi hopes to achieve this through illegal, coercive, aggressive, and deceptive actions that undermine Taiwan's will to resist," they said, adding: "Xi remains willing to use military force to achieve his goal."

While the United States "must address the massive backlog in weapons deliveries to Taiwan," Taiwan also needed to "step up with us," it said.

The letter was addressed to the leaders of Taiwan's opposition parties, Kuomintang (KMT) and Taiwan People's Party (TPP), as well as the KMT's parliamentary speaker and Democratic Progressive Party's (DPP) caucus whip.

The lawmakers praised Taiwan's "important progress" in strengthening its defenses, but said "we fear that without significant increases in Taiwan's defense spending at levels reflected in President Lai's proposed special budget, this progress will be insufficient," it said.

The United States has long been Taiwan's most important backer and biggest arms provider, and the democratic island would be heavily reliant on US support in a potential conflict with China.

Xi has warned Washington against selling weapons to Taiwan.

Asked about the letter, KMT spokesman Niu Hsu-ting said the party would "respond appropriately" but insisted "we will absolutely not compromise on oversight of the budget."

TPP chairman Huang Kuo-chang told reporters that his party "must also fulfil our responsibility to safeguard the public interest and provide oversight to ensure that the money is spent where it matters most."

Lai told AFP in an exclusive interview on Tuesday that he was confident the defense budget would be passed.

"In a democratic society, every political party is ultimately accountable to the people," said Lai, who belongs to the DPP.

The TPP caucus did a sudden U-turn earlier this week, agreeing to send the government's version of the special defense spending bill to committee for joint review.

But KMT chairperson Cheng Li-wun has vowed that her party "will not relent."

As well as the government's version, lawmakers will also consider the TPP's stripped-down version of the defense bill that allocates $12.6 billion for military purchases.