US President Joe Biden is holding the first ever summit with the leaders of Japan and the Philippines Thursday to show support for Manila amid tensions with China -- as Beijing said it was being "smeared" by its rivals.

Biden's three-way meeting with Japanese Prime Minister Fumio Kishida and Philippines President Ferdinand Marcos follows repeated confrontations between Chinese and Philippine vessels in the disputed South China Sea.

China claims almost the entirety of the crucial waterway, brushing aside competing claims from several Southeast Asian nations including the Philippines, as well as an international ruling that has declared its stance baseless.

"President Marcos is coming under pressure from the PRC's aggressive tactics," a senior US administration official told reporters, using the acronym for the People's Republic of China.

"What you'll see is a clear demonstration of support and resolve from both President Biden and Prime Minister Kishida. We stand shoulder to shoulder with Marcos."

The Japanese premier is already in Washington following a lavish state visit on Wednesday during which he and Biden unveiled a historic upgrade in defense ties.

Kishida gave a joint address to Congress on Thursday in which he urged Americans to overcome their "self-doubt" about their role as a global power.

Warning of risks from the rise of China, Kishida said that Japan -- stripped of its right to a military after World War II -- was determined to do more to share responsibility with its ally the United States.

At the trilateral summit, the United States, Japan and the Philippines are expected to announce new joint naval exercises along with Australia, similar to drills they had in the region at the weekend, officials said.

They are also set to unveil new economic cooperation measures.

Biden, 81, and Marcos, 66 -- who is seen as closer to Washington than his more authoritarian and China-leaning predecessor Rodrigo Duterte -- will also hold separate talks on Thursday.

China charges smear campaign

China said that the United States and Japan had "smeared" its reputation during Kishida's state visit.

Beijing foreign ministry spokeswoman Mao Ning said Washington and Tokyo had "attacked China on Taiwan and maritime issues, grossly interfered in China's internal affairs, and seriously violated the basic norms governing international relations."

The United States has a mutual defense pact with Manila and has repeatedly declared its "ironclad" commitment to defending the Philippines against an armed attack in the South China Sea.

Tensions have become particularly acute around the Second Thomas Shoal, a remote reef in the Spratly Islands. 

Biden's commitment to the Philippines was "clear" and he had "repeated many times" that Washington's defense treaty applied to the South China Sea, a second US official said.

The trilateral summit is part of Biden's efforts to seal alliances with like-minded nations in a region that both Beijing and Washington consider part of their geopolitical backyard.

Japan and the Philippines are the latest Asia-Pacific allies to be hosted by Biden, who was joined by Kishida and South Korean President Yoon Suk Yeol at Camp David in August.

Biden has also moved to manage tensions with China, holding a two-hour phone call with President Xi Jinping last week following a face-to-face meeting in San Francisco in November.

On Wednesday Biden said the major upgrade in defense ties with Japan was "purely defensive" and "not aimed at any one nation or a threat to the region."