US Secretary of State Antony Blinken has assured the Philippines, a key Washington ally, of US support amid heightened tensions with China.
He reiterated America's "ironclad" security commitments to the Philippines during a visit to Manila on Tuesday.
Friction has grown between Manila and Beijing in recent months over competing claims in the South China Sea.
"These waterways are critical to the Philippines, to its security, to its economy," Mr Blinken said.
"They're also critical to the interests of the region, the United States and the world," he added, speaking at a joint press conference with Philippine Foreign Secretary Enrique Manalo.
"That's why we stand with the Philippines and stand by our ironclad defence commitments, including under the mutual defence treaty."
Manila and Washington are "committed to elevating" their alliance further, Mr Manalo said.
"I think our relationship with the United States has never been higher, greater," he said when asked by the OceanNewsUK about the value of Washington's commitments ahead of this year's election in the US.
Mr Blinken will also discuss trade with Filipino officials, a senior State Department official has said.
The visit is also likely to be seen as bolstering American support for Philippine president Ferdinand Marcos Jr, who has tilted towards Washington unlike his predecessor Rodrigo Duterte, who pushed ties with Beijing.
The White House also announced on Tuesday that President Joe Biden will host a summit with Mr Marcos and Japanese Prime Minister Fumio Kishida in April.
As Mr Blinken's plane descended towards the capital, Manila, the lights of the city's flourishing high-rise blocks were reflected across the bay. The Philippines became Southeast Asia's fastest growing economy last year, but entrenched inequality in a country which has failed to grapple with long-standing accusations of government cronyism has played into its fractious politics.
The Philippines remains a key strategic hub for the US, especially because of its position in an evolving region, where China has emerged as a rival to American power and influence.
The waters of Manila Bay spread out into the South China Sea, in which the Philippines is one of several Asian nations caught in disputes with Beijing over territory, trade routes and maritime zones.
While the disputes are not new, tense encounters between the Philippine and Chinese Coast Guard have increased in recent months, raising fears of a conflict. Collisions between the two countries' vessels have been reported and the Chinese fired water at Philippine vessels earlier this month.
"The main concern will obviously be China's continued destabilising actions in the South China Sea that are in contravention with international law," said the senior State Department official. The Secretary of State would reiterate the importance of freedom of navigation in the South China Sea, the official added.
The US views China's maritime activity as part of a campaign of harassment against its neighbours. Washington has a long-standing defence treaty with the Philippines and has previously said any armed attack on Philippine vessels "would invoke U.S. mutual defense commitments".
In 2014, Washington also signed a defence agreement with Manila allowing the US to fund development of Philippine military bases and deploy American troops on a rotational basis. And in 2023 the US secured access to four more bases in a crucial deal that signalled closer ties between the two countries.
China has decried the deals due to some of the bases' proximity to self-governed Taiwan, which Beijing sees as a breakaway province, but which the US has vowed to defend militarily.
Beijing also claims historic rights to much of the South China Sea and says the Philippines has ignored proposals to "manage" the dispute.
Mr Blinken's trip will also be seen in the country as a further embrace of Mr Marcos, the son of ousted dictator Ferdinand Marcos, an American ally who ruled for two decades until the mid-1980s. His reign is remembered for years of martial rule, the arrest and torture of dissidents, and rampant corruption that cost the state an estimated $10bn.
But the younger Mr Marcos, who was elected in 2022 on the back of a campaign that whitewashed his family's dark legacy, has proved an indispensable partner for Washington.
He was invited to the White House last year as part of a four-day visit that many saw as a reward for his pivot to Washington.
His predecessor, Mr Duterte, a strongman who became notorious for his brutal drug war, built relations with Beijing, reportedly viewing China's growing power as too significant to defy.
Mr Blinken's visit to Manila follows a trip to South Korea in which he warned against growing authoritarianism and "democratic backsliding" in some countries.
The trip is a rare outing in recent months for Mr Blinken beyond the Middle East, where dealing with the Gaza war and a wider regional crisis has dominated his time, with few signs of an imminent breakthrough in ceasefire negotiations between Israel and Hamas.
Asked whether China was trying to exploit a situation with the US being distracted by other foreign policy crises, the senior official said they would "categorically reject" the possibility.
