Speaking in Manila after meeting with Philippine President Rodrigo Duterte, Pompeo said Beijing's artificial islands in waters also claimed by the Southeast Asian nation and other neighbours was a threat.

"China's island-building and military activities in the South China Sea threaten your sovereignty, security and therefore economic livelihood, as well as that of the United States," he said at a joint news conference with Philippine Foreign Secretary Teodoro Locsin.

"As the South China Sea is part of the Pacific, any armed attack of Philippine forces, aircraft or public vessels in the South China Sea will trigger mutual defence obligations under Article 4 of our Mutual Defence Treaty."

Pompeo's comments marked the first time any US official had publicly stated Washington's intent to defend its poorly-armed ally specifically in the flashpoint sea.

A 1951 US-Philippine mutual defence treaty committed Manila and its former colonial master to come to each other's aid in case of an "armed attack in the Pacific area" on either party.

Senior Duterte officials have called for a review of the pact because they were unsure whether it applied to the maritime row.

'Have your back' 

Philippine troops and fishermen have frequently complained about harassment by Chinese maritime security forces around some of the islands and reefs Manila occupies.

The United States has said it is not taking sides in the dispute over waters claimed by China, the Philippines, Brunei, Malaysia, Taiwan and Vietnam.

However, Washington has asserted its right to freely sail over waters through which trillions of dollars in global trade pass through each year and which reputedly contain vast mineral and oil reserves.

The Philippines used to be the staunchest critic of China's expansive claims over the sea. 

But after his election in 2016 Duterte put the dispute on the back burner in favour of courting Chinese trade and investment.

He threatened a split with the United States and called then President Barack Obama a "son of a whore".

Relations are being rebuilt under US President Donald Trump, who has hailed Duterte's actions -- including a drugs crackdown that has claimed thousands of lives -- as a sign of toughness.

Locsin on Friday downplayed his government's suggestions for a review of the defence pact, saying in its "vagueness lies the best deterrence". 

"We are very assured, we are very confident that United States has, in the words of Secretary Pompeo and words of President Trump to our president: we have your back," Locsin said.

 'Huawei risks'

Doubling down on China, Pompeo also warned the Philippines and other nations against using technology from Chinese telecoms giant Huawei.  

The Philippines' Globe Telecom plans to roll out with Huawei commercial 5G services this year, while the Duterte government last year signed an $400 million deal for Huawei to install closed-circuit television cameras in two Philippine cities to deter crime. 

US officials suspect that Beijing could use the Shenzhen-based Huawei's products to spy on Western governments. The company denies the allegations. 

On Friday, Pompeo said using Huawei technology carried risks. 

"We want to make sure that the world has their eyes wide open as to the risks of having that technology be part of the infrastructure or backbone or networks," Pompeo said. 

"We believe that competition, whether it's in 5G or some other technology, opt to be open, free, transparent and we worry that Huawei is not that." 

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