Stage set for Trump-Putin finale

"Hopefully, someday, maybe he'll be a friend," Trump added.

"Maybe we [the US] will get along with the group [Russia] that we're protecting [Europe] against. I think that's a real possibility," he said.

He downplayed the Helsinki summit, saying: "I think we go into that meeting not looking for so much".

He noted that talks would cover Putin's war in Syria, his other war in Ukraine, Russia's annexation of Crimea, and its meddling in the 2016 US election.

But he indicated he would give Putin an easy ride on election-meddling, at least.

"It's one of those things. All I can do is say: 'Did you?' and 'Don't do it again.' But he may deny it," Trump said, amid an US investigation into whether he himself colluded with Putin to influence the 2016 vote.

Trump also indicated he might let bygones be bygones on Crimea.

"They [Russia] built bridges to Crimea. They just opened a big bridge that was started years ago. They built, I think, a submarine port, substantially added billions of dollars," Trump said, praising Putin's investments there.

Trump, one day earlier, had signed a Nato declaration on non-recognition of the "illegal" annexation, but he said he felt free to change his mind.

"What will happen with Crimea from this point on? That I can't tell you," the US leader said.

The Helsinki talks culminate a week of Trump's bull-in-a-China-shop diplomacy in Europe.

He began by goring German leader Angela Merkel on her plans to build a Russia gas pipeline and on defence spending at a Nato summit in Brussels.

He even threatened to pull the US out of the Western alliance in what Merkel described as "intense" talks.

He then attacked British prime minister Theresa May while arriving in London, saying that her rival, Boris Johnson, might make a better PM, and that there would be no UK-US trade deal under May's Brexit plan.

Finland braces

The spectacle of Nato and EU infighting will make Putin look strong when he meets Trump on Monday, on the heels of the World Cup final in Moscow on Sunday.

The venue, Helsinki, also frames the talks in historic grandeur, recalling Cold War-era summits in the Finnish capital.

Finnish authorities have registered almost 1,500 journalists from 61 countries to cover the event, including from as far afield as Algeria, the Philippines, and Taiwan.

They also plan to introduce checks on their EU borders in a nationwide security scramble.

The Finlandia Hall congress centre, where the two leaders and up to 2,000 of their officials and diplomats will meet, has been freshly painted.

The Hotel Kamp in the city centre has also unfurled a banner, which says: "Keep Peace - that's how we say 'cheers' or 'na zdorovje' in Finnish."

But few Finns, and few Europeans, expected much good to come out of the event as preparations came to a head.

Just four percent of Finns said Trump had made the world a safer place in a survey for the Yle news agency.

That antipathy will be on show in four or more anti-Trump protests in Finland, following similar ones in the UK on Friday.

Most of his supporters had low levels of education and voted for far-right parties, the Yle survey found, highlighting a social divide in Europe.

Step backward?

"His behaviour ... is another profoundly disturbing signal that the president is more loyal to president Putin than to our Nato allies," two senators from the opposition Democratic Party in the US, Nancy Pelosi and Chuck Schumer, said on Trump's conduct at the Nato summit, articulating broad Western concern.

"If the president leaves the Putin meeting without ironclad assurances and concrete steps toward a full cessation of Russian attacks on our democracy, this meeting will not only be a failure - it will be a grave step backward for the future of the international order," they said.