US President-elect Joe Biden says the White House has so far been "sincere" in helping his transition to power.

"It has not been begrudging so far, and I don't expect it to be," he told NBC News in an interview.

He spoke as he unveiled his choice of top officials for when he takes over from Donald Trump in January.

Trump finally agreed to allow the formal transition process to begin on Monday, nearly three weeks after the presidential election.

Yet he still refuses to admit defeat, repeating unsubstantiated claims that the 3 November vote was "rigged".