The United States has a decades-old security alliance with the Philippines that includes a mutual defence treaty and a 2014 pact which allows the US military to store equipment on nine Philippine bases.

With US president-elect Trump assuming office on January 20, many in the Philippines are wary on the future of Manila and Washington's relationship.

But Marcos "expressed optimism in maintaining the strong and dynamic relationship" between the two countries -- particularly on economics, defence and security -- during a phone call with outgoing Vice President Kamala Harris.

"The progress that we have made is terribly encouraging and we just hope to build on that," said a statement from Manila's Presidential Communications Office, quoting Marcos during the call.

He also hopes the two countries "continue to stand for our shared values and the rule of international law".

The call with Harris came the same week Marcos held a call with outgoing US President Joe Biden and Japanese Prime Minister Shigeru Ishiba where they reaffirmed their commitment to "advance a free and open Indo-Pacific", said a White House statement Monday.

The Philippines and Japan -- both longtime US allies -- have stepped up security cooperation with Washington to blunt China's efforts to control most of the disputed South China Sea.

In her call with Marcos, Harris echoed Biden to emphasise the "critical nature of (the trilateral cooperation) to maintain security of the South China Sea", the Manila statement said, quoting the US vice president.

The trilateral alliance "is a very important way to deepen our economic cooperation and build secured supply chains as well as promote security across the region", Harris said.