President Donald Trump says the US relationship with Russia "has never been worse" as he prepares for a high-stakes summit with Russia's Vladimir Putin.

Key points: Donald Trump says the EU is a "foe" of the US

Donald Trump says the EU is a "foe" of the US Donald Tusk says "America and the EU are best friends"

Donald Tusk says "America and the EU are best friends" Trump says he has low expectations of summit with Vladimir Putin

Mr Trump used Twitter to blame the hostilities on, "many years of US foolishness and stupidity and now, the Rigged Witch Hunt!"

That's Mr Trump's favourite derogatory term for the special counsel investigation into Russian election meddling and possible ties to his campaign that has already led to a slew of indictments, including of a dozen Russian intelligence officers last week.

Mr Trump also claimed credit for bolstering NATO as he headed into a day of meetings with the Russian President.

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He rocked a NATO summit last week when he lambasted members for failing to meet defence spending pledges and questioned the value of the alliance, before doing a 180 and embracing it.

But today, Mr Trump said in a tweet he had received, "many calls from leaders of NATO countries" thanking him for helping, "to get them focused on financial obligations, both present & future".

European Union an enemy, says Trump

Mr Trump earlier named the European Union as a top adversary and again denounced the news media as the "enemy of the people" before arriving in Helsinki for his high-stakes summit with Russia's Vladimir Putin.

Mr Trump and his top aides were downplaying expectations for Monday's summit as Mr Trump continued to rattle allies by lumping the EU in with Russia and China as a "foe" of the US.

In an interview with CBS News conducted at his Scottish golf resort, Mr Trump named the EU, which includes many of America's closest allies, at the top of his list of biggest global foes.

"I think the European Union is a foe, what they do to us in trade," Mr Trump said, adding that "you wouldn't think of the European Union, but they're a foe".

European Council President Donald Tusk quickly responded on Twitter, using one of Mr Trump's favourite stock phrases.

"America and the EU are best friends. Whoever says we are foes is spreading fake news," Mr Tusk wrote.

Mr Trump and his wife Melania flew from Glasgow, Scotland to Helsinki late on Sunday. ( AP: Pablo Martinez Monsivais )

Russia and China also 'foes' but only in certain aspects

Mr Trump said that Russia was a foe "in certain respects" and that China was a foe "economically … but that doesn't mean they are bad. It doesn't mean anything. It means that they are competitive."

The US President has been reluctant to criticise Mr Putin over the years and has described him in recent days not as an enemy but as a competitor.

On Sunday, Mr Trump flew to Finland, the final stop on a week-long trip that began last Tuesday.

Near Mr Trump's hotel, police roped off a group of about 60 mostly male pro-Trump demonstrators waving American flags.

Chants of "We love Trump, We love Trump" broke out as the President's motorcade passed, and Mr Trump waved.

Mr Trump set expectations for the summit low, telling CBS News, "I don't expect anything … I go in with very low expectations."

Protesters have already taken to the streets of Helsinki ahead of the Putin-Trump summit. ( AP: Markus Schreiber )

The US has no extradition treaty with Moscow and cannot compel Russia to hand over citizens. Russia's constitution prohibits extraditing its citizens to foreign countries.

Contradicting Mr Trump in an interview on American ABC's This Week program, US national security adviser John Bolton said the idea of asking Mr Putin to turn over the 12 military intelligence officials was "pretty silly" and argued that doing so would put the US President in a "weak position".

In the CBS News interview, Mr Trump declined to discuss his goals for the summit. "I'll let you know after the meeting," he said.

He cited his historic meeting with North Korean leader Kim Jong-un in June as a "good thing", along with meetings he had with Chinese President Xi Jinping.

"Nothing bad is going to come out of" the Helsinki meeting, he said, "and maybe some good will come out".

From aboard Air Force One, Mr Trump complained in tweets that he wasn't getting enough credit for his meeting with Mr Kim and railed that "Much of our news media is indeed the enemy of the people" as he headed to sit down with Mr Putin.

"If I was given the great city of Moscow as retribution for all of the sins and evils committed by Russia over the years, I would return to criticism that it wasn't good enough — that I should have gotten Saint Petersburg in addition!" he tweeted.

Sorry, this video has expired Donald Trump and Vladimir Putin meet at the G20 summit in Hamburg last year.

AP/Reuters