US President Barack Obama on Tuesday slammed Donald Trump's "flattery" of Russia's Vladimir Putin as "unprecedented" and "out of step" with both Democrats and rank-and-file Republicans.The Republican presidential nominee has repeatedly voiced admiration for the Russian president - notably calling him a better leader than Obama - and advocated a US rapprochement with Moscow.Trump's support for Russia comes at a time of escalating tensions, as Washington and Moscow lock horns over the Syria conflict, and US officials accuse Russia of directing cyber attacks aimed at interfering in November's election."Mr Trump's continued flattery of Mr Putin and the degree to which he appears to model many of his policies and approach to politics on Mr Putin is unprecedented in American politics," Obama told a joint news conference with Italian Prime Minister Matteo Renzi.The US leader said he had pursued a strategy of "constructive" engagement with Russia - be it on the conflict in Ukraine, or Syria -and refuted Trump's assertion that the current administration failed to stand up to Moscow on the world stage."Any characterization that somehow we have improperly challenged Russian aggression or have somehow tried to encroach on their legitimate interests is just wrong," Obama said.Under the next president, he insisted, the United States must continue to "call out" Moscow over behavior that violates international norms."The bottom line is that 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," Obama said."But their behavior has undermined international norms, and international rules, in ways that we have to call them out on, and anybody who occupies this office should feel the same way.""We can't go around talking about human rights or freedom of the press or democracy or freedom of religion, or non-discrimination or basic laws on war or the sovereignty or territorial integrity of countries, no matter how small, and then extol the virtues of somebody who violates those principles," Obama said."Mr Trump rarely surprises me these days," the president added."I'm 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."