U.S. President Joe Biden said on Thursday, April 11, that the United States' defense commitment to Indo-Pacific allies Japan and the Philippines was "ironclad."

He, Fumio Kishida, and Ferdinand Marcos Jr. gathered in the White House to discuss economic and defensive partnerships amid growing concern about provocative Chinese military action in the region.

"The United States defense commitments to Japan and to the Philippines are ironclad. They're ironclad," he said in his initial remarks before the closed-door meeting. "As I said before, any attack on Philippine aircraft, vessels or armed forces in the South China Sea would invoke our mutual defense treaty."

The U.S. and the Philippines have had a mutual defense treaty for 70 years. Before that, the tropical archipelago nation was part of Washington's Pacific holdings in the early 20th century.

Biden's forceful reinforcement of the American commitment came amid persistent skirmishes between the Philippine and Chinese coast guards in the disputed South China Sea.

The White House organized the first-ever trilateral summit with Japan and the Philippines as a potent response to China's attempts at "intimidation," saying that it would send a message that China was "the outlier in the neighborhood," a Biden administration official said. 

Recently, the three nations and Australia conducted a maritime patrol in the South China Sea in response to the harassment of Chinese coast guard ships in the area, which caused some injuries to Filipino supply boat crews. 

"Today's summit is an opportunity to define the future that we want, and how we intend to achieve it together," Marcos said.

HNGN earlier reported that Japan and the U.S. have also upgraded their military and diplomatic cooperation due to the Chinese threat.

Biden, Marcos, and Kishida have also announced more joint patrols in the Indo-Pacific this year, including the U.S. Coast Guard welcoming its Filipino and Japanese counterparts onto a USCG vessel during a future patrol for training, the White House added.

Philippine president Marcos also called China out in a Fall of Bataan Day speech this week, saying that his nation would stand against oppression amid  "unacceptable" and "unjust" threats to the country's sovereign rights. 

In response to the summit, Chinese officials criticized Washington for allegedly exacerbating tensions, the Associated Press reported. 

"No one should violate China's territorial sovereignty and maritime rights and interests, and China remains steadfast in safeguarding our lawful rights," Chinese foreign ministry spokeswoman Mao Ning said on Thursday.