A cooperation agreement by the Philippines, the United States and Japan will change the dynamic in the South China Sea and the region, Philippine President Ferdinand Marcos Jr. said on Friday, while seeking to assure China it was not a target.

Biden meets with Japan Prime Minister Kishida and Philippine President Marcos Jr. in Washington.

"I think the trilateral agreement is extremely important," Marcos told a press conference in Washington a day after meeting President Joe Biden and Japanese Prime Minister Fumio Kishida in the nations' first trilateral summit.

"It is going to change the dynamic, the dynamic that we see in the region, in ASEAN in Asia, around the South China Sea," Marcos said, referring to the Association of Southeast Asian Nations.

The three leaders expressed "serious concerns" about China's "dangerous and aggressive behavior" in the South China Sea, a conduit for more than $3 trillion of annual ship-borne commerce with various maritime disputes among China and other countries.

Still, Marcos said the summit was "not against any country" but had focused on deepening economic and security relations among Manila, Washington and Tokyo.

China claims almost the entire South China Sea despite a 2016 ruling by the Permanent Court of Arbitration found Beijing's sweeping claims had no legal basis.

Philippine and Chinese ships have had a series of run-ins in the past month that included the use of water cannon and heated verbal exchanges.

Beijing on Thursday summoned Manila's ambassador to the country and a Japanese embassy official to oppose what its foreign ministry described as "negative comments" against China.

The deepening China-Philippines row coincides with an increase in security engagements with the United States under Marcos, including expansion of U.S. access to Philippine bases, as well as with Japan, which is expected to sign a reciprocal troop pact with Manila.

Biden has asked Congress for an additional $128 million for infrastructure projects at the Philippine bases.

Marcos also expressed confidence that around $100 billion in possible investment deals over the next five to 10 years from the summit will come into fruition.

While in Washington, Marcos also met with Defense Secretary Lloyd Austin, who assured him of continued U.S. support.

"This whole cooperation is critical to our collective security and continued prosperity across the region," Austin said, reiterating Biden's strong defense commitment.