United States President Barack Obama on Tuesday called Republican presidential nominee Donald Trump's admiration of Russian President Vladimir Putin "unprecedented" in American politics and accused Republicans of softening stand on Russia.

According to the outgoing US President, Trump's continued flattery of Putin and the degree to which he appears to model many of his policies and approaches to politics on Putin, is unprecedented in American politics.

"It is out of step with, not just what Democrats think but out of step with what, up until the last few months, almost every Republican thought, including some of the ones who are now endorsing Trump," he said.

Trump, he noted, rarely surprises him these days.

"I am much more surprised and troubled by the fact that you have Republican officials who historically have been adamantly anti-Russian and in fact have attacked me for even engaging them diplomatically now supporting and, in some cases, echoing his positions. It's quite a reversal," he said.

"We think that Russia is a large important country with a military that is second only to ours, and has to be a part of the solution on the world stage, rather than part of the problem.

"But their behavior has undermined international norms and international rules in ways that we have to call them out on," Obama told reporters at a joint news conference with the visiting Italian Prime Minister Matteo Renzi, in the Rose Garden of the White House.

"Anybody who occupies this office should feel the same way because these are values that we fought for and we protected," he said.