The United States has an "unwavering" commitment to the Philippines, US Vice President Kamala Harris told the country's president Monday during a visit aimed at countering China and rebuilding ties that were fractured over human rights abuses in the Southeast Asian nation.

Harris is the highest-ranking US official to visit Manila since President Ferdinand Marcos took power in June, signaling a growing rapport between the longtime allies after years of frosty relations under his Beijing-friendly predecessor Rodrigo Duterte.

She also met with her Philippine counterpart Sara Duterte, the daughter of the former leader whose deadly drug war sparked an international investigation into alleged human rights abuses.

"We stand with you in defense of international rules and norms as it relates to the South China Sea," Harris told Marcos at the start of talks in the presidential palace in Manila.

"An attack on the Philippine armed forces, public vessels, or aircraft in the South China Sea would invoke the US mutual defense commitment… that is our unwavering commitment to the Philippines."

Marcos said he did not "see a future for the Philippines that does not include the United States."

The United States has a long and complex relationship with the Philippines — and the Marcos family. Marcos's dictator father ruled the former US colony for two decades with the support of Washington, which saw him as a Cold War ally.

Relations between the two countries soured under the foul-mouthed Duterte. In 2016, Duterte called Barack Obama a "son of a whore" over warnings he would be questioned by the then US president over his controversial drug war.

Washington is now seeking to bolster its security alliance with Manila under the new president.

That includes a mutual defense treaty and a 2014 pact, known by the acronym EDCA, which allows for the US military to store defense equipment and supplies on five Philippine bases.

It also allows US troops to rotate through those military bases.

EDCA stalled under Duterte but the United States and the Philippines have expressed support for accelerating its implementation as China becomes increasingly assertive.

"We have identified new locations and have begun a process with the Philippines to finalize those," a US official told reporters on condition of anonymity ahead of Harris's meeting with Marcos.

On Tuesday, Harris will visit the Philippine island province of Palawan, which lies along hotly contested waters in the South China Sea.

China claims sovereignty over almost the entire sea, while the Philippines, Vietnam, Malaysia, and Brunei have overlapping claims to parts of it.

Beijing has ignored a 2016 international tribunal ruling that its claims have no legal basis.

Harris will meet members of the Philippine Coast Guard on board one of the country's two biggest coast guard vessels and deliver a speech.

US commitment

Harris's trip to the Philippines is part of US efforts to remove any doubt about its commitment to the Asia-Pacific as China aggressively expands its regional influence.

It comes after Harris and US President Joe Biden met separately with Chinese President Xi Jinping last week.

Harris reinforced Biden's message that "we must maintain open lines of communication to responsibly manage the competition between our countries" while speaking to Xi on the sidelines of the Asia-Pacific Economic Cooperation forum in Bangkok, a White House official said.

While her trip to Palawan would likely annoy China, the United States had more to gain from sending a message of reassurance to the Philippines, said Greg Poling, director of the US-based Asia Maritime Transparency Initiative.

"The Philippines will be much more reassured than China will be irritated," Poling said.

Among the initiatives to be launched during Harris's trip are negotiations for a civilian nuclear pact between the United States and the Philippines.

That could lead to the future sales of US nuclear reactors to the Southeast Asian country.

Marcos is a strong supporter of renewable energy and has insisted on the need to reconsider building nuclear power plants in the disaster-prone country.

However, before the United States can sell nuclear equipment to the Philippines, the two countries must sign a civilian nuclear pact known as a "123 agreement", which is designed to prevent the spread of nuclear weapons.