US President-elect Donald Trump has announced that he is nominating Republican Senator and noted China hawk Marco Rubio to be his secretary of state, as he quickly fills out his cabinet ahead of his January 20 inauguration.

Trump, who clashed bitterly with Rubio for the presidential nomination in 2016, said in a statement nominating him: "Marco is a Highly Respected Leader, and a very powerful Voice for Freedom."

"He will be a strong Advocate for our Nation, a true friend to our Allies, and a fearless Warrior who will never back down to our adversaries," Trump said.

Rubio said in a statement that he will work with Trump "every day to carry out his foreign policy agenda."

"Under the leadership of President Trump, we will deliver peace through strength and always put the interests of Americans and America above all else," Rubio said in a statement.

Rubio, the son of Cuban immigrants to Miami and vociferous opponent of Latin American communists, China and Iran, would be the first Hispanic as the top US diplomat.

He had sponsored the Hong Kong Human Rights and Democracy Act of 2019 to sanction SAR and central government officials, and has in turn been sanctioned by Beijing.

Rubio was among a flurry of new nominations announced by Trump. He also tabbed Republican lawmaker Matt Gaetz to be attorney general, and former Democratic representative Tulsi Gabbard as his director of national intelligence.

"I know Tulsi will bring the fearless spirit that has defined her illustrious career to our intelligence community, championing our constitutional rights and securing peace through strength," Trump said in a statement.

He also extolled Gaetz, a staunch ally who has defended Trump in his legal battles and impeachment fights.

"Few issues in America are more important than ending the partisan Weaponization of our Justice System," Trump posted on social media. "Matt will end Weaponized Government... and restore Americans' badly-shattered Faith and Confidence in the Justice Department."

As attorney general, Gaetz would drive all aspects of the work of the US Justice Department, which for years has carried out an investigation into sex trafficking and obstruction of justice allegations involving him.

Gaetz, who denies all wrongdoing, was told last year that there would be no charges against him, but he remains the subject of a House ethics investigation.

Republican and Democratic senators immediately voiced doubts that Gaetz's nomination would survive the confirmation process, which can involve intense questioning during difficult, fraught hearings. (Agencies)