Marcos, the son and namesake of the late Philippine dictator, secured more than half of the votes in Monday's poll to win the presidency by a wide margin and cap a remarkable comeback for his family.

He and running mate Sara Duterte, who also won the vice presidential race in a landslide, have embraced key policies of outgoing President Rodrigo Duterte, including his position on China.

The elder Duterte sought to pivot away from the US, the Philippines' former colonial master, toward China since taking power in 2016 – and appeared reluctant to confront Beijing over territorial disputes in the South China Sea.

Even before Marcos declared victory, the US and Chinese presidents were quick off the mark to start building a personal relationship seen as strategically vital by both rivals.

In a phone call on Wednesday, Chinese President Xi Jinping told Marcos the two countries had been "partners through thick and thin", Chinese state television reported on Thursday.

"I attach great importance to the development of China-Philippines relations and am willing to establish a good working relationship with President-elect Marcos, adhere to good neighborliness and friendship," Xi said.

'Friend, ally, partner'

The US said it will seek close security ties with the Philippines under Marcos, but made clear it would raise human rights.

In a phone call, US President Joe Biden congratulated Marcos and said he wanted to expand cooperation on a range of issues, including climate change and "respect for human rights".

Marcos said on Thursday he had assured Biden that the Philippines "always held the United States in high regard as a friend, an ally, and a partner".

Marcos also invited Biden to his June 30 inauguration, but did not say if the US leader had accepted.

The US has a complex relationship with the Philippines – and the Marcos family.

After ruling the former US colony for two decades with the support of the US, which saw him as a Cold War ally, Marcos senior went into exile in Hawaii in the face of mass protests and with the nudging of Washington in 1986.

As regional tensions remain high, Washington is keen to preserve its security alliance with Manila that includes a mutual defense treaty and permission for the US military to store defense equipment and supplies on several Philippine bases.

But Marcos's spokesman Vic Rodriguez warned a US contempt of court judgment against the younger Marcos could "affect" the relationship.

The decades-old issue relates to the family's failure to pay compensation to thousands of victims of human rights abuses during Marcos senior's regime.

'Engagement' with China

Under Duterte, Manila's previously frosty relations with Beijing warmed as the authoritarian firebrand set aside an international ruling on the South China Sea in exchange for promises of trade and investment.

China claims almost the entirety of the waterway and has ignored the 2016 ruling by The Hague-based Permanent Court of Arbitration that its historical claim is without basis.

It has reinforced its stance by building artificial islands over some contested reefs and installing weapons on them.

Brunei, Malaysia, Taiwan and Vietnam also have competing claims in the waters, which are believed to hold rich deposits of oil and gas.

Ahead of the elections, Marcos said he would seek "engagement" with China rather than confrontation over their rival claims in the South China Sea.

"We won't solve our problem with China if we fight them," Marcos said in February.

"President Duterte's engagement approach is correct because in my opinion that is the only way to resolving our conflicting claims with China."

But Duterte has faced domestic pressure to take a harder line on China's growing assertiveness in the South China Sea and in the past year insisted his country's sovereignty over the waters is not negotiable.

In July, Duterte walked back on a decision to ax a key military deal – the Visiting Forces Agreement – with the US during a visit by Pentagon chief Lloyd Austin.