The Pentagon chief made the remarks at an annual security forum in Singapore, as the administration of US President Donald Trump spars with Beijing on trade, technology, and influence over strategic corners of the globe.

Since taking office in January, Trump has launched a punishing trade war with China, sought to curb its access to key AI technologies, and deepened security ties with allies such as the Philippines, which is engaged in escalating territorial disputes with Beijing.

"The threat China poses is real and it could be imminent," Hegseth said at the Shangri-La Dialogue attended by defence officials from around the world.

Beijing is "credibly preparing to potentially use military force to alter the balance of power in the Indo-Pacific", he added.

Hegseth warned the Chinese military was building the capabilities to invade Taiwan and "rehearsing for the real deal".

Beijing has ramped up military pressure on Taiwan and held multiple large-scale exercises around the island, which are often described as preparations for a blockade or invasion.

The United States was "reorienting toward deterring aggression by communist China", Hegseth said, calling on US allies and partners in Asia to swiftly upgrade their defences in the face of mounting threats.

- 'Wake-up call' -

Hegseth described China's conduct as a "wake-up call", accusing Beijing of endangering lives with cyber attacks, harassing its neighbours and "illegally seizing and militarising lands" in the South China Sea.

Beijing claims almost the entire disputed waterway, through which more than 60 percent of global maritime trade passes, despite an international ruling that its assertion has no merit.

It has clashed repeatedly with the Philippines in the strategic waters in recent months, with the flashpoint set to dominate discussions at the Singapore defence forum, according to US officials.

"China's assertiveness in the South China Sea has only increased in recent years," Casey Mace, charge d'affaires at the US embassy in Singapore, told journalists ahead of the meet.

"I think that this type of forum is exactly the type of forum where we need to have an exchange on that."

But Beijing has not sent any top Chinese defence ministry officials to the summit, dispatching a delegation from the People's Liberation Army National Defence University instead.

Hegseth's comments came after Trump stoked new trade tensions with China, arguing that Beijing had "violated" a deal to de-escalate tariffs as the two sides appeared deadlocked in negotiations.

The world's two biggest economies had agreed to temporarily lower eye-watering tariffs they had imposed on each other, pausing them for 90 days.

But on Friday, Trump wrote on his Truth Social platform: "China, perhaps not surprisingly to some, HAS TOTALLY VIOLATED ITS AGREEMENT WITH US," without providing further details.

Asked about the statement on CNBC, US Trade Representative Jamieson Greer took aim at Beijing for continuing to "slow down and choke off things like critical minerals".

He added that the United States' trade deficit with China "continues to be enormous", and that Washington was not seeing major shifts in Beijing's behaviour.

- 'Priority theatre' -

The Indo-Pacific is "America's priority theatre", the US defence chief said, pledging to ensure "China cannot dominate us -- or our allies and partners".

He said the United States had stepped up cooperation with allies including the Philippines and Japan, and reiterated Trump's vow that "China will not invade (Taiwan) on his watch".

But he called on US allies and partners in the region to ramp up spending on their militaries and "quickly upgrade their own defences".

"Asian allies should look to countries in Europe for a newfound example," Hegseth said, citing pledges by NATO members including Germany to move toward Trump's spending target of five percent of GDP.

"Deterrence doesn't come on the cheap."