Wilmington, Delaware - In a high-stakes gaffe, US President Joe Biden has been caught on a hot mic telling Quad leaders on Saturday that China is "testing" the US and its allies.

During a carefully curated farewell summit for President Joe Biden including Quad leaders from Australia, India, and Japan, Biden was heard criticizing Beijing for being aggressive in the Indo-Pacific.

"China continues to behave aggressively, testing us all across the region," Biden was heard saying in an unintentional hot mic moment.

"And it's true in the South China Sea, the East China Sea, South China, South Asia, and the Taiwan Straits."

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The summit was also attended by Indian Prime Minister Narendra Modi, Japanese Prime Minister Fumio Kishida, and Australian Prime Minister Anthony Albanese, each of whom were present during the gaffe.

President Biden's comments came as he opened the farewell summit, and undercut an attempt by the Quad to avoid directly mentioning China.

According to Biden, China's President Xi Jinping has been "looking to buy himself some diplomatic space, in my view, to aggressively pursue China's interest" despite domestic policy problems at home.

The US and China have been at loggerheads in recent months over what Washington sees as a growing assertiveness both around the Taiwan issue and in the South China Sea.

Recent clashes between the Philippines and Chinese coast guards have made US-China relations particularly frosty, despite efforts earlier this year to defrost communications.

Biden's comments risk undermining not only an attempt to reconfigure diplomatic channels but also make it clear that the Quad is about more than just pushing back against Beijing.

A joint statement issued by the four leaders at the Wilmington, Delaware summit painted a positive image of the Quad as "a force for good that delivers real, positive, and enduring impact" in the Indo-Pacific.

"Anchored by shared values, we seek to uphold the international order based on the rule of law," the statement, which was issued by the White House, said.

"We reaffirm our steadfast commitment to a free and open Indo-Pacific that is inclusive and resilient."

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The "Wilmington Declaration Joint Statement" set forth nine priorities for the Quad leaders, focusing on issues like regional food security, health, humanitarian assistance, maritime security, and more.

Careful not to mention China by name, the Quad announced a "Quad-at-Sea Ship Observer Mission" for 2025, which will see the coast guards of all four countries join forces to "improve interoperability and advance maritime safety."

Quad leaders did affirm, however, a commitment to addressing issues in the East and South China Seas. Again, they did so without directly mentioning China.

"We are seriously concerned about the situation in the East and South China Seas," the statement read. "We continue to express our serious concern about the militarization of disputed features, and coercive and intimidating maneuvers in the South China Sea."

"We condemn the dangerous use of coast guard and maritime militia vessels, including increasing use of dangerous maneuvers," the statement added.