US House of Representatives leader Paul Ryan and other Republican leaders are scratching their heads over Donald Trump's recent praise for Russian President Vladimir Putin.

Trump made said in a political forum that he thought Putin was a strong leader. The Kremlin will not respond to the comments by the Republican candidate which have sparked concern among GOP grandees.

"Vladimir Putin is an aggressor who does not share our interests," Ryan said at a news conference, adding: "It certainly appears that he is conducting state-sponsored cyber attacks on what appears to be our political system.

"That is not acting in our interests, and that is an adversarial stance."

In a national defence forum on NBC, Trump said Putin had been a leader "far more than our president," and praised Putin's "great control" of his nation.

A startled Republican South Carolina Senator and former presidential candidate Lindsey Graham, responding to his party's candidate, told reporters: "If you're running for leader of the free world and you're expressing admiration for Putin, well then you're losing me."

He said of Putin: "Other than destroying every instrument of democracy in his own country, having opposition people killed, dismembering neighbors through military force, and being the benefactor of the butcher of Damascus, he's a good guy."

Comparison with Hitler

Just to make certain he was perfectly clear, he added, according to Bloomberg: "I think this is the biggest miscalculation since people thought Hitler was a good guy."

Marco Rubio
Senator Marco Rubio expressed concern at Trump's Putin commentsCarlos Barria/ Reuters

Conservative Florida Republican Senator also a former presidential candidate, told the Guardian: "I have tremendous policy disagreements with President Obama, but Vladimir Putin is an authoritarian thug who is accountable to no one. I don't think what Vladimir Putin exhibits is leadership. I think what he exhibits is thuggery, and we should be clear-eyed about that."

"Murderer and a thug" was a common assessment of Putin by other Republicans on Capital Hill. No one else preferred him to Obama.

Democrats were particularly horrified. Hillary Clinton called Trump's praise for Putin "scary" and Obama called it "wacky."

"I don't think the guy's qualified to be president of the United States and every time he speaks, that opinion is confirmed," Obama said.

But the Trump campaign stood by the position. Trump hasn't responded to the criticism nor expressed any changes in his perspective. And vice presidential candidate Mike Pence told CNN: "It's inarguable that Vladimir Putin has been a stronger leader in his country than Barack Obama has been in this country.

"And that's going to change the day that Donald Trump becomes president."