U.S. House Speaker Paul Ryan is refusing to support Donald Trump as the Republican nominee for president, insisting Thursday that the businessman must do more to unify the GOP.

The surprise declaration from Ryan on CNN's The Lead amounted to a stunning rebuke of Trump from the Republican Party's highest-ranking officeholder.

"I'm just not ready to do that at this point. I'm not there right now," the Wisconsin representative said. "And I hope to. And I want to, but I think what is required is that we unify this party."

In a statement, Trump responded that he himself isn't ready to support Ryan's agenda, either.

"Perhaps in the future we can work together and come to an agreement about what is best for the American people," Trump said. "They have been treated so badly for so long that it is about time for politicians to put them first!"

Even in an election cycle that's exposed extreme and very public divisions within the GOP, Ryan's decision to withhold his support from Trump was remarkable, as the GOP's top elected leader, second in line to the presidency, turned his back on his own party's presumptive nominee.

Donald Trump waves after speaking at a campaign victory party after Ted Cruz dropped out of the race for the Republican presidential nominee following the results of the Indiana state primary. John Kasich suspended his campaign the next day, leaving Trump as the presumptive nominee. (Lucas Jackson/Reuters)

Ryan had maintained his silence since Trump effectively clinched the nomination with a commanding win in Indiana on Tuesday that forced his two remaining rivals from the race. Other Republican leaders, including Senate Majority Mitch McConnell, offered their grudging support for Trump, and Ryan had seemed likely to eventually do the same.

Instead he balked in comments that could also reflect concern for his own political future and potential run for president in 2020.

"We will need a standard-bearer that can unify all Republicans, all conservatives, all wings of our party, and then go to the country with an appealing agenda," Ryan said. "And we have work to do on this front, and I think our nominee has to lead in that effort."

Pair meeting next week

Ryan's announcement sent shockwaves through the Republican establishment. Republican National Committee Chairman Reince Priebus, who is close with the speaker, got no advance warning before Ryan's comments were made public.

Later Thursday, Priebus said he had spoken to both men and expected them to work out their differences. In an interview recorded for Fox News Channel's Hannity, he said Ryan and Trump "are meeting next week to talk about these things."

The highly unusual salvos between the likely White House nominee and the House speaker came at a moment when all involved would normally be turning from the intraparty warfare of the primaries to unifying the party for November's elections. Instead, the Republican Party remains asunder, with prospects for coming together uncertain.

Ryan made clear he won't be supporting Hillary Clinton and wants to come around to backing Trump. And he acknowledged the import of Trump's victories over a field of some of the GOP's most experienced politicians, saying the mogul had "tapped into something in this country that was very powerful. And people are sending a message to Washington that we need to learn from and listen to."

Ryan, his party's 2012 vice-presidential nominee, had been seen as a possible "white knight" candidate who could emerge as an alternative to Trump at a contested convention. He called a press conference last month to rule himself out, and Trump now looks set to gather the 1,237 delegate votes needed to clinch the nomination ahead of the July gathering in Cleveland, foreclosing the contested convention scenario.

Ryan will serve as the convention's chairman, presiding over portions of the proceedings that will elevate Trump to the official status of nominee.

Trump and Ryan have publicly clashed in the past. Ryan rebuked Trump for plans to bar Muslims from the country, and when Trump was slow to disavow former Ku Klux Klan leader David Duke. Trump told a crowd in South Carolina in February that Ryan doomed the GOP presidential ticket four years ago by saying entitlement programs need reform.