"What we've seen, and what is of real concern to us, is increasingly aggressive actions by the government in Beijing directed at Taiwan, raising tensions in the (Taiwan) Straits," Blinken told NBC's "Meet the Press."

He added that the United States has a long-standing commitment to Taiwan "to make sure that Taiwan has the ability to defend itself, and to make sure that we're sustaining peace and security in the Western Pacific."

Blinken would not be drawn on whether Washington would respond militarily to any Chinese action involving Taiwan.

But he added: "All I can tell you is it would be a serious mistake for anyone to try to change the existing status quo by force."

The United States has been concerned for some time about the buildup of Chinese forces in the region, including aggressive actions in the region's oceans and military overflights in skies near Taiwan.

Taiwan, a self-governing democracy, is considered by Beijing a territory awaiting reunification, by force if necessary.

Earlier on Sunday, the Philippine armed forces said it will hold joint exercises with hundreds of US soldiers over the next two weeks, its military chief said Sunday, amid growing tensions with Beijing in the South China Sea. 

The annual war games between the military allies were cancelled in 2020 due to the coronavirus pandemic. 

This year's event will be on a smaller scale than in previous years due to the health crisis, Philippine military chief Lieutenant General Cirilito Sobejana told AFP. 

About 700 US soldiers and up to 1,300 members of the Philippine military will take part -- about a quarter of the usual attendance, Sobejana said.

"The exercise this year is a hybrid of virtual and physical activities," he said.

"It's a low-key exercise, just to keep the alliance -- the contact -- between the two armed forces."

The US embassy in Manila did not immediately respond to a request for comment. 

The announcement came hours after a phone call between US Defence Secretary Lloyd Austin and his Philippine counterpart Delfin Lorenzana, who last week tested positive for the coronavirus. 

They "discussed the situation in the South China Sea, and the recent massing of People's Republic of China maritime militia vessels at Whitsun Reef," according to a readout provided by the Pentagon. 

To deepen their defence cooperation, Austin proposed "enhancing situational awareness of threats in the South China Sea". The readout did not elaborate.

Tensions over the resource-rich sea have intensified since hundreds of Chinese vessels were detected last month at Whitsun Reef, which is in the Spratly Islands where several countries, including China and the Philippines, have rival claims.

China, which claims almost the entirety of the sea, has refused repeated appeals by the Philippines to withdraw the vessels, which Manila says unlawfully entered its exclusive economic zone.

Beijing said previously they were fishing vessels sheltering from bad weather.

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The United States reminded China last week of Washington's treaty obligations to the Philippines in the event of an attack in the waters.

"An armed attack against the Philippines' armed forces, public vessels or aircraft in the Pacific, including in the South China Sea, will trigger our obligations under the US-Philippines Mutual Defense Treaty," State Department spokesman Ned Price told reporters.

The resumption of joint military drills comes more than a year after Philippine President Rodrigo Duterte -- who has pivoted towards China since taking power in 2016 -- gave notice that he planned to axe the Visiting Forces Agreement.

The plan to break the deal -- central to hundreds of joint military exercises with the US every year and a major component of their nearly 70-year-old alliance -- has been suspended.

But it has underscored the complicated relations between the Philippines and its former colonial master, the United States. 

Duterte's stance also raised concern that the regional balance of power could tilt in Beijing's favour.