Washington, United States: Republican presidential nominee Donald Trump said Tuesday he was "not quite" ready to endorse House Speaker Paul Ryan in his upcoming primary, a stunning rebuke to the party's top elected official.

With little more than three months before the White House election Trump, in an interview with The Washington Post, also said he was not prepared to endorse Republican Senator John McCain in his re-election bid in Arizona.

The real-estate tycoon also lashed out at another Republican US senator, Kelly Ayotte of New Hampshire, as weak and disloyal.

"I like Paul, but these are horrible times for our country," Trump said of Ryan, who faces a challenge by a conservative insurgent in the August 9 Wisconsin primary.

"We need very, very strong leadership. And I'm just not quite there yet," Trump said. "I'm not quite there yet."

Trump's refusal appears to amount to blunt political retribution three months after Ryan used very similar language to say "I'm just not ready" to back Trump. That controversial declaration set off weeks of debate among conservatives over whether to rally behind the presidential hopeful.

Ryan eventually endorsed Trump despite his misgivings.

Now only two weeks after its national convention in Cleveland aimed at showcasing Republican unity, the party faces further divisions, which come amid heated debate over Trump's comments about the Muslim American parents of a US Army captain killed in combat in 2004.

Ryan's challenger for his House of Representatives seat, Paul Nehlen, defended Trump's comments, earning praise from Trump, who on Monday night tweeted he "very much appreciated" Nehlen's "kind words."

Trump in the Post interview did not say whether he would endorse either candidate but said "I'm giving very serious consideration to that whole situation, to Ryan, to Paul."

Trump also said Ryan had sought his endorsement, a claim disputed by the speaker's campaign.

"Neither Speaker Ryan nor anyone on his team has ever asked for Donald Trump's endorsement," Ryan campaign spokesman Zack Roday said in a statement, adding the campaign was "confident" Ryan would win his primary.

Trump was adamant about not having endorsed McCain, a former prisoner of war who issued a scathing anti-Trump statement about his handling of the Khan family controversy.

"I've never been there with John McCain because I've always felt that he should have done a much better job for the vets," Trump said.

"He has not done a good job for the vets," he added. "Our vets are not being treated properly. They're not being treated fairly."

McCain, seeking a sixth Senate term, faces a three-way battle in his August 30 primary.

Ayotte, facing a tough re-election battle in New Hampshire, also has spoken out harshly against Trump, saying she was "appalled" by his attacks on the Khans whose son was killed in Iraq.

Trump's criticism of her was severe.

"Kelly Ayotte has given me zero support, and I'm doing great in New Hampshire," he said.

"We need loyal people in this country. We need fighters in this country. We don't need weak people. We have enough of them."