WASHINGTON (AFP) - US House Speaker Paul Ryan, the top Republican in Congress, said on Thursday he was not ready to support Donald Trump as the party's presumptive presidential nominee in November's general election.

"To be perfectly candid with you, I'm not ready to do that yet," Ryan told CNN in a bombshell interview that heightened concerns about whether conservatives will be able to rally around Trump in his expected matchup against Democrat Hillary Clinton.

Ryan, the Republican vice-presidential nominee in 2012, stressed that he hoped he would be able to support Trump in the future, provided the brash billionaire is able to show leadership in unifying the party.

"He's got some work to do," Ryan said, noting that the burden was on Trump to begin the healing process after a brutal primary campaign and Trump's long string of insulting remarks.

Ryan has expressed criticism of Trump before. But Thursday's comments were all the more startling because Trump has now emerged as the party's standardbearer and Ryan, as speaker of the House of Representatives, will oversee the Republican presidential nominating convention in July.

"I think that he needs to do more to unify the party... then to go forward and appeal to all Americans from every walk of life and background, and a majority of independents," Ryan said.

He insisted, however, that no Republicans should support Clinton - as several have pledged to do after Trump's rivals dropped out.

"To be the party and climb the final hill and win, we need a standardbearer that can unify all - all conservatives and the wings of the party - and then go to the country with an appealing agenda," he said.

"The nominee has to lead in that effort."

Both Bush presidents - George H.W. Bush and his son George W. Bush - have signalled they will not endorse Trump in 2016, while Mitt Romney, the 2012 nominee on the ticket with Ryan, is reportedly not going to attend the Republican convention.