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.

China's embassy in Singapore blasted the speech, accusing Washington of escalating tensions in the region and "profiting from creating risks of war".

Trump has launched a trade war with China since taking office in January, has 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 said.

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

China has ramped up military pressure on Taiwan and held large-scale exercises around the island that 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.

- 'Stirring up trouble' -

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 disputed South China Sea.

Beijing claims almost the entire 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 forum, according to US officials.

As Hegseth spoke in Singapore, China's military announced that its navy and air force were carrying out routine "combat readiness patrols" around the Scarborough Shoal, a chain of reefs and rocks Beijing disputes with the Philippines.

Hegseth's hard-hitting address drew a critical reaction from the Chinese embassy in the city-state.

"The speech is steeped in provocations and instigation," the embassy wrote on its Facebook page.

"Mr Hegseth repeatedly smeared and attacked China and relentlessly played up the so-called 'China threat'," the statement said, accusing the United States of being the "biggest troublemaker" in the region.

Beijing did not send any top defence ministry officials to the summit, dispatching instead a delegation from the People's Liberation Army National Defence University led by Rear Admiral Hu Gangfeng.

Without referring to Hegseth by name, Hu said of his speech that "these actions are essentially about stirring up trouble, creating division, inciting confrontation, and destabilising the Asia-Pacific".

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.

- 'Cannot dominate' -

Reassuring US allies on Saturday, Hegseth said the Asia-Pacific region was "America's priority theatre", 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".

However, he called on US 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 towards Trump's defence spending target of five percent of GDP.

"Deterrence doesn't come on the cheap."