US President Donald Trump has said the future of Western civilisation is at stake as he affirmed his commitment to NATO's Article 5 during a speech in central Warsaw.

Key points: Western interests were being tested by "propaganda, financial crimes and cyber warfare", US leader says

Western interests were being tested by "propaganda, financial crimes and cyber warfare", US leader says US has demonstrated its commitment to NATO with actions, Mr Trump says

US has demonstrated its commitment to NATO with actions, Mr Trump says Mr Trump stops short of condemning Russia for meddling in the US election

Visiting the Polish capital on the eve of his first meeting with Russian President Vladimir Putin, Mr Trump vowed to confront "new forms of aggression" targeting the West and called for Moscow to stop fomenting unrest around the world.

Yet he pointedly again stopped short of condemning Russia for meddling in the US election.

Buoyed by an electrified crowd in Poland chanting his name, Mr Trump warned that Western interests were being tested by "propaganda, financial crimes and cyber warfare", forcing NATO to adapt.

"We urge Russia to cease its destabilising activities in Ukraine and elsewhere, and its support for hostile regimes including Syria and Iran, and to join the community of responsible nations in our fight against common enemies and in defence of civilisation itself," Mr Trump said.

"The fundamental question of our time is whether the West has the will to survive."

Opening his second overseas trip as President, Mr Trump also warned North Korea that he is considering "some pretty severe things" in response to the isolated nation's unprecedented launch of a missile capable of reaching the US.

Mr Trump also stated unequivocally that the US stands "firmly behind Article 5", the NATO provision requiring the US to defend other member nations if they come under attack.

On his inaugural trip to Europe as president in May, Mr Trump declined to affirm that commitment, to the dismay of US allies who said it cast doubt on his allegiance to the alliance.

Mr Trump told the crowd in Warsaw's Krasinski Square that the United States "has demonstrated not merely with words, but with its actions that we stand firmly" behind Article 5, "the mutual defence commitment".

"Words are easy, but actions are what matters," he said after earlier calling on all NATO members to commit to their defence funding pledges.

Article 5 has only been invoked once, after the September 11, 2001 terrorist attacks in the United States.

Sorry, this video has expired Donald Trump called on Russia to stop its destabilizing activities in Ukraine. (Photo: AP/Alik Keplicz)

'Nobody knows for sure' if Russia hacked election: Trump

Mr Trump's critique of threats to the West did not appear to extend to Russia's actions last year during the presidential campaign.

In a news conference before his speech, Mr Trump questioned the veracity of American intelligence about foreign meddling in the US election, arguing that Russia was not the only country that may have interfered.

"Nobody really knows for sure," Mr Trump said.

As US investigations into Russia's meddling forge ahead, Mr Trump is under intense scrutiny for how he handles his first face-to-face session with Mr Putin.

US intelligence officials say the Russia leader ordered interference into the 2016 election that brought Mr Trump to the White House.

Mr Trump and Mr Putin plan to sit down in Hamburg, Germany, on the sidelines of the G20 summit.

Asked specifically whether he planned to discuss election meddling with Mr Putin, Mr Trump demurred.

Loath to cast a shadow on his election victory, Mr Trump has avoided firmly blaming Moscow for campaign hacking in the past, and was similarly elusive in Poland.

He argued variably that it could have been Russia, probably was Russia and indeed was Russia, while insisting it could have been other countries, too, and adding: "I won't be specific."

AP