One day before his first meeting with Russian President Vladimir Putin, U.S. President Donald Trump said on Thursday that no one knows for sure whether Moscow intervened in the 2016 U.S. election but that he suspected Russian involvement.

Speaking to reporters in Poland, Trump played down the assessment of his own intelligence agencies that Russia meddled in the election by hacking Democrats' emails and distributing online propaganda.

“I think it was Russia, but I think it was probably other people and/or countries, and I see nothing wrong with that statement. Nobody really knows. Nobody really knows for sure,” Trump told a news conference.

Investigations by a special counsel, Robert Mueller, and several U.S. congressional committees are looking into whether Russia interfered in the election and colluded with Trump's campaign. Those probes are focused almost exclusively on Moscow’s actions, lawmakers and intelligence officials say, and no evidence has surfaced publicly implicating other countries.

Moscow has denied any interference, and Trump says his campaign did not collude with Russia.

Trump, who defeated Democrat Hillary Clinton in the November election, will meet Putin on Friday at a G20 summit in the German city of Hamburg for their first official encounter.

It was not clear whether the Republican president would bring up the issue of election interference when the two men meet.

In a speech in Warsaw, Trump affirmed U.S. commitment to the defence of NATO allies and gently criticized Russia.

Trump frequently praised Putin as a strong leader during the election campaign and called for better U.S. relations with Moscow but he has since tempered that rhetoric.

Representative Adam Schiff, the top Democrat on the House of Representatives' Intelligence Committee, said Trump's remarks in Poland about the election only propagated "his own personal fiction."

"The President's comments today, again casting doubt on whether Russia was behind the blatant interference in our election and suggesting – his own intelligence agencies to the contrary – that nobody really knows, continue to directly undermine U.S. interests," Schiff said in a statement.

Trump cast doubt at his news conference over media reports that all 17 U.S. intelligence agencies concluded in January that Russia had meddled in the election.

"Let me just start off by saying I heard it was 17 agencies. I said, boy, that's a lot, do we even have that many intelligence agencies, right? Let's check it. And we did some very heavy research. It turned out to be three or four. It wasn’t 17," Trump said.

The U.S. intelligence report in January accusing Russia of meddling in the U.S. election was issued by the Office of the Director of National Intelligence, which oversees the 17 U.S. intelligence agencies.

But the report acknowledges that it is based on an assessment by the CIA, the Federal Bureau of Investigation and the National Security Agency, which held the key classified information on Russian activities.

Trump said the CIA told his predecessor, Democratic President Barack Obama, last August about Russian interference in the election but he did nothing to stop it.

"I think what happened is he thought Hillary Clinton was going to win the election, and he said let's not do anything about it. Had he thought the other way, he would have done something about it," Trump said.