WASHINGTON 

The US's commitment to the Philippines' security is "ironclad" amid mounting tensions with China, President Joe Biden said Monday as he hosted his Philippine counterpart. 

"We are facing new challenges, and I can't think of a better partner to have than you," Biden said alongside President Ferdinand Marcos Jr. during brief remarks in the Oval Office before the leaders went into closed-door meetings.

"The United States also remains ironclad in our commitment to the defense of the Philippines, including in the South China Sea, and we are going to continue to support the Philippines' military modernization goals," he added.

Biden's meeting with Marcos Jr. comes amid rising tensions in the South China Sea between the island nation and China, which claims the disputed waters as its territory.

The Philippines on Friday accused China of “aggressive tactics” at sea after a Chinese Coast Guard ship blocked a Philippine patrol vessel in a disputed area of the South China Sea last week that the Philippines says is part of its exclusive economic zone.

The US on Saturday issued its full-throated support for Manila, saying any attack on the Philippines armed forces in the Pacific would invoke mutual defense assurances, prompting a response from the American military.

Marcos Jr. said the Philippines is in "arguably the most complicated geopolitical situation in the world right now," emphasizing the need for the nations to "strengthen our alliance and our partnership."

"It is only natural for the Philippines to look to its sole treaty partner in the world to strengthen and to redefine the relationship that we have, and the roles that we play, in the face of those rising tensions that we see now around the South China Sea and Asia-Pacific and Indo-Pacific regions," he said.

The high-level meeting comes as the US seeks to bolster its regional alliances and partnerships amid worsening US-Sino relations.

In a joint statement released following their meeting, Biden and Marcos Jr. emphasized "their unwavering commitment to freedom of navigation and overflight in the South China Sea, and the importance of respecting the sovereign rights of states within their exclusive economic zones consistent with international law."

They further reiterated Washington's mutual defense commitments to the Philippines and welcomed cooperation with additional nations "that share the United States’ and the Philippines’ commitment to international law and mutual respect."

"They look forward to establishing trilateral modes of cooperation among the Philippines, Japan and the United States, as well as the Philippines, Australia and the United States," the leaders said.