President-elect Joe Biden's cabinet nominees promised Tuesday that the United States would stay tough on China but vowed a new era of international cooperation after Donald Trump's divisive "America First" approach.

A day before Biden becomes the 46th president, Antony Blinken, his choice for secretary of state, indicated that the new administration will revive agreements with Iran and Russia but described both countries as threats.

"Not one of the big challenges we face can be met by one country acting alone -- even one as powerful as the US," Blinken, a mild-mannered longtime aide to Biden, told his Senate confirmation hearing.

"We can revitalize our core alliances –- force multipliers of our influence around the world. Together, we are far better positioned to counter threats posed by Russia, Iran, and North Korea and to stand up for democracy and human rights."

Blinken distanced himself from the outgoing president's needling of allies and denunciations of multilateralism but said that Trump "was right in taking a tougher approach to China."

Blinken backed the determination Tuesday by the outgoing secretary of state, Mike Pompeo, that China was committing genocide against Uighurs and other mostly Muslim people.

Retired general Lloyd Austin, the nominee to be defense secretary, told his hearing that the rising Asian power "constitutes a significant and long-term security threat to the United States and to our allies and partners."

He also pledged to tackle extremists in the military's own ranks, after some members of the military dressed in civilian clothing took part in the attack on the US Capitol by a pro-Trump mob. 

His comments came as 12 members of the National Guard force deployed to protect Biden's inauguration were removed during a sweep of background checks to root out any members with potential links to extremist groups.

Biden's pick for intelligence chief said she would remain apolitical. Avril Haines, a former top CIA official nominated to be director of national intelligence (DNI), also said the US espionage community would step up its focus on China as a key threat.

She also promised senators she would release a still-classified report into Saudi Arabia's assassination of Jamal Khashoggi, a US-resident Saudi journalist.

He faced friendly questioning even from most Republicans, indicating he is likely to win quick confirmation in the Senate.

A deputy secretary of state in Barack Obama's presidency, Blinken promised to return to diplomacy including on Iran but acknowledged it's "destabilizing activities" in the region.

Blinken said that Biden would also move quickly to extend the New START treaty on nuclear reduction, the last remaining arms pact with Russia, which expires on February 5.

In another shift, Blinken said that he would immediately review Pompeo's designation of Yemen's Huthi rebels as a terrorist group.

Blinken said that Saudi Arabia, a close ally of Trump, bore much of the responsibility for what the United Nations calls the world's worst humanitarian crisis.