China's accelerated military modernisation poses a clear and growing threat to Taiwan, and US intervention might only risk intensifying pressure from Beijing, US defence officials say.

Washington's top US military officer in Asia-Pacific Admiral Philip Davidson made waves last week by warning that Beijing could seek to invade Taiwan within six years.

And as Pentagon chief Lloyd Austin makes Asia the target of his first overseas trip, US defence officials have hammered home the threat they see posed by Beijing's rapid military build-up.

"Preparing for Taiwan contingencies has been a focus in China's military modernisation for some time," acting assistant secretary of defense for Indo-Pacific security affairs David Helvey told reporters as Austin headed to Tokyo on Monday.

"So as their capabilities are increasing, obviously we are paying very careful attention to the military balance in the Taiwan strait."

Beijing's ambitions in the region are expected to top Defense Secretary Austin's agenda in talks with allies in Tokyo, Seoul and New Delhi.

Beijing claims Taiwan and surrounding waters as its own territory, and last week Davidson warned the threat of invasion there was "manifest during this decade, in fact, in the next six years".

Beijing dismissed those claims, but a senior US defence official pointed Monday to China's efforts to step up its naval capacity, despite the disruption of the coronavirus pandemic.

"Just in 2020, in the midst of all the Covid-related stuff, China still commissioned 25 major new ships -- and these were not tug boats or patrol boats," the official said, speaking on condition of anonymity.

"These were cruisers, and destroyers and frigates and amphibs and ballistic-missile submarines."

Democratic and self-ruled Taiwan split from China at the end of a civil war in 1949 and exists under the constant threat of invasion by the mainland.

Washington switched diplomatic recognition from Taiwan to China in 1979, but remains the island's most important unofficial ally and military backer.

There is a balance

The official said there are several "things that are not definitive but play into this issue of timing" in terms of the threat to Taiwan from Beijing.

"The first one is the announcement late last year of 2027 as a new interim milestone for PLA (People's Liberation Army) modernisation," he said, noting that it brought the previous timeline forward by eight years.

President Xi Jinping's third term is also expected to end in 2027, he added, calling it a mistake to buy into Chinese messaging on the issue.

"We are deceiving ourselves and we run the risk of falling into a misdirection of Beijing" by doing so, he said.

Beijing accused Davidson of attempting to "hype up" the threat of invasion to inflate Washington's defence spending and justify its own military posture in Asia.

And a second senior US defence official told reporters Monday that Washington would take a measured approach to the issue.

"There is a balance," he said. "We do not view a violent invasion of Taiwan as good for anybody."

But, "militarily, we know that if we do too much, push too hard, China will use that optic and they'll do more against Taiwan".

President Joe Biden's administration has also dispatched Secretary of State Antony Blinken to the region for talks with allies alongside Austin.

And last week leaders of the so-called Quad, a grouping of the United States, Australia, Japan and India, held talks again putting the spotlight on their calls for a "free and open Indo-Pacific".

The United States also sent a warship through the Taiwan Strait last week, just a day after Davidson's comments, the third such voyage since Biden took office.