"Someone said 'is he an enemy?' No, he's not my enemy. Is he your friend? No, I don't know him well enough, but the couple of times I've gotten to meet him we got on very well.

"But ultimately he's a competitor. He's representing Russia, I'm representing the United States," Trump added. "Hopefully some day he'll be a friend, I just don't know."

Trump said he would be discussing the civil war in Syria, the conflict in Ukraine, as well as allegations of Russian meddling in the 2016 US election.

"I will be asking about meddling, your favourite question," he told reporters during a press conference on the sidelines of a NATO summit in Brussels.

"All I can do is say 'did you?' and 'don't do it again'. He may deny," Trump added.

The US president was also asked if he would be prepared to recognise Crimea as part of Russia after it was annexed from Ukraine by Moscow in 2014.

Some news reports and analysts have suggested Trump might be prepared to concede the territory to Putin in exchange for cooperation in Syria.

"What will happen to Crimea from this point on? That I can't tell you, but I'm not happy about Crimea."

Trump blamed his predecessor Barack Obama "who allowed it to happen."

"That was on Barack Obama's watch, that was not on Trump's watch. Would I have allowed it to happen? No, I would not."

He also said that he expected to discuss NATO military exercises near the Russia border in the Baltic Sea, which Moscow considers provocative.

European nations are desperate for the US to maintain its military commitment to continue defending the continent under the NATO military alliance.

"We'll be talking about it," Trump said.