US Secretary of State Antony Blinken is due to hold talks with Philippine President Ferdinand Marcos on Tuesday, as the countries seek to reinforce security and economic ties in the face of growing Chinese aggression.

Blinken's visit to Manila -- his second since Marcos took office in June 2022 -- is part of a brief Asia tour that also included South Korea.

Washington is keen to improve relations with its regional allies, such as Manila and Seoul, in a bid to deter China and North Korea as tensions rise in the South China Sea over Taiwan and on the Korean peninsula.

Blinken's trip comes ahead of a trilateral meeting in Washington next month between US President Joe Biden, Marcos and Japanese Prime Minister Fumio Kishida.

Announcing the three-way summit with the Asia Pacific allies, White House Press Secretary Karine Jean-Pierre said the leaders would push a "shared vision for a free and open Indo-Pacific".

During his meeting with Marcos and his Philippine counterpart Enrique Manalo on Tuesday, Blinken is expected to reaffirm the United States' security commitments to the Philippines.

"What we're trying to demonstrate... is that we are committed to the Indo-Pacific, to this region, despite everything else that's going on in the world right now," a senior State Department official told reporters on Monday.

"I would expect when we're in Manila, the main concern will obviously be China's continued destabilising actions in the South China Sea that are in contravention of international law."

Top US officials have repeatedly said that "an armed attack" against Philippine public vessels, aircraft, armed forces and coastguard would invoke the 1951 US-Philippines mutual defence treaty in which Washington is obliged to defend its ally.

Blinken's visit follows a spate of incidents involving Philippine and Chinese vessels in the South China Sea, including collisions, in recent months.

Marcos has expressed "great alarm" over the confrontations that have sparked diplomatic sparring between Manila and Beijing. 

'Advancing the alliance'

Beijing has accused Washington of using the Philippines as a "pawn" in the dispute over the waterway and various reefs.

China claims almost the entire South China Sea, brushing aside competing claims from several Southeast Asian nations, including the Philippines, and an international ruling that has declared its stance baseless.

Blinken's visit was aimed at "advancing the alliance", said Renato De Castro, professor for international studies at De La Salle University in Manila.

Marcos would also likely be interested to hear from Blinken what are the "trigger points" for the United States to invoke their mutual defence treaty, said Herman Kraft, a political science professor at the University of the Philippines.

Relations between Washington and Manila were rocky under former president Rodrigo Duterte, who pivoted towards China and away from the United States.

Since Marcos took power, however, they have warmed as he seeks to deepen cooperation with the United States and regional neighbours, while standing up to Chinese aggression towards Philippine vessels in the South China Sea.