The top elected US Republican, Paul Ryan, says he is not ready to endorse Donald Trump, a sign of the challenges the party's presumptive presidential nominee faces rallying the Republican establishment behind his White House bid.

Ryan, the speaker of the US House of Representatives, said on Thursday that conservatives wanted to know if Trump shares their values.

"I hope to support our nominee, I hope to support his candidacy fully," Ryan said on CNN. "At this point, I'm just not there right now."

Ryan previously criticised Trump for saying that if he did not win the nomination, his supporters might riot at the Republican National Convention in July.

Other Republicans grappled this week with how robustly to support a candidate who shuns the party line on trade and has upset the party establishment with offensive comments about women and immigrants.

Ryan said he hoped the party would be unified by this summer, "but I think a lot of the burden is on the presumptive nominee to do that and so we'll see."

"He won fair and square," Ryan said of Trump, acknowledging his own policy differences with the New York billionaire businessman. He added: "If we don't unify all wings of the party, we're not going to win this election."

Trump's remaining rivals in the Republican race dropped out this week, clearing his path to be picked as the presidential nominee. He will likely face Democratic front-runner Hillary Clinton in the November 8 general election.