Washington DC - On his first full day in office, Secretary of State Marco Rubio issued a not-so-subtle warning to China over its actions at sea.

Rubio on Tuesday met in Washington with his counterparts from Japan, India, and Australia – the so-called Quad.

In a joint statement, they promised to work toward a "free and open Indo-Pacific," a term often used to mean countering China's assertiveness in its own region.

The four said they support a region "where the rule of law, democratic values, sovereignty and territorial integrity are upheld and defended."

Joe Biden Joe Biden and wife Jill post farewell selfie ahead of Trump inauguration: "We love you, America"

"We also strongly oppose any unilateral actions that seek to change the status quo by force or coercion," the statement said.

The ministers confirmed they would work to hold a Quad summit previously scheduled for this year in India, which would mean an early trip by Trump to the increasingly close US partner.

Rubio also met separately with each minister. With Japanese Foreign Minister Takeshi Iwaya, Rubio discussed North Korea and "joint efforts against China's destabilizing actions," State Department spokeswoman Tammy Bruce said.

China has increasingly clashed with the Philippines, a US ally, over territorial disputes.

Rubio in his confirmation hearing also vowed to deter China against an invasion of Taiwan, the self-governing island it claims as its own.

During his presidential campaign, Trump rattled Taiwan by saying it needed to pay the US protection money, and Trump's nominee for defense secretary, Pete Hegseth, made headlines at his confirmation hearing by not being able to name a member of the Association of Southeast Asian Nations (ASEAN).

The Quad was envisioned by late Japanese prime minister Shinzō Abe and expanded into a leaders' summit by former President Joe Biden.

China has repeatedly lashed out at the alliance, saying it is a US plot to militarily encircle the rising Asian power.

Rubio, a three-term Florida senator who a day earlier was unanimously confirmed by his peers, arrived at the State Department with a promise to defend US diplomats while also pursuing Trump's belief in "America First."

"I expect every nation on earth to advance their national interests. And in those instances – and I hope there will be many – in which our national interests and theirs align, we look forward to working with them," Rubio said.

"We recognize that there will be those times unfortunately as humans interact with one another because of our nature that there will be conflict," he added.

Donald Trump Trump targets diversity programs and transgender community with early executive orders

"We will seek to prevent them and avoid them, but never at the expense of our national security."

With Trump's return, a slew of senior career diplomats quit their posts at the State Department as the new administration pushes to bring in political appointees it sees as loyal.

Addressing employees with his wife and four children by his side, Rubio said: "There will be changes."

"But the changes are not meant to be destructive, they're not meant to be punitive," he insisted.

"But we need to move faster than we ever have because the world is changing faster than we ever have."