House Speaker Paul Ryan says he is "not ready" to endorse presumptive Republican nominee Donald Trump.
"I'm just not ready to do that at this point," he said of publicly backing Trump. "I'm not there right now."
Ryan said in an interview with CNN that he "hopes" to eventually feel ready to back Trump.
"I think what is required is that we unify this party. And I think the bulk of the burden on unifying the party will have to come from our presumptive nominee."
Ryan added that Trump needs to "do more to unify this party, to bring all wings of the Republican Party together."
Trump responded in a statement Thursday afternoon, saying "I am not ready to support Speaker Ryan's agenda."
"Perhaps in the future we can work together and come to an agreement about what is best for the American people. They have been treated so badly for so long that it is about time for politicians to put them first!" he said.
Trump's campaign manager was more blunt.
Asked at a rally later Thursday if it stung to have the head of the Republican Party withhold his endorsement, manager Corey Lewandowski pointed to the stage where Trump was speaking and said, "That's the leader of the Republican Party."
In the interview with CNN, Ryan appeared to question Trump's commitment to conservative values but reiterated that he hopes to help the party coalesce around its leadership.
"There are lots of questions that conservatives, I think, are going to want answers to, myself included," he said.
Ryan added that Republicans should not consider likely Democratic nominee Hillary Clinton as an alternative.
"No Republican should ever think about supporting Hillary Clinton, let me make that clear, but for us to be a successful party and to climb that final hill and win the presidency, we will need a standard bearer that can unify all Republicans, all conservatives, all wings of the party."
The Wisconsin lawmaker was viewed as a possible savior of the party establishment before ruling out a late entry to the presidential race in April.
Mitt Romney, who selected Ryan as his running mate in 2012, has also been silent on a Trump endorsement to date. A spokesman for the former GOP nominee said Thursday that he has no plans of attending this summer's Republican convention.