US Vice President Mike Pence on Saturday pledged an "unwavering" commitment to transatlantic ties, aiming to reassure allies including German Chancellor Angela Merkel, who pleaded with nations not to go it alone.

Capping a week of whirlwind diplomacy by American officials who have descended on Europe to calm nerves rattled by President Donald Trump, Pence underlined the United States' loyalty to its old friends.

"The United States is and will always be your greatest ally. Be assured that President Trump and our people are truly devoted to our transatlantic union," he told European leaders at the Munich Security Conference.

But he also reiterated a call for Europe's NATO allies to step up defence spending to reach the two percent of gross domestic product target agreed by the alliance.

"The promise to share the burden of our defence has gone unfulfilled for too many for too long, and it erodes the very foundation of our alliance," he warned, adding that "the time has come to do more".

At the same time, he did not go further and threaten, as Trump had done, to walk away from the alliance if fellow members failed to meet the target.

The US, he said, will boost defence spending significantly "to defend our nation and our treaty allies from the known threats of today and the unknown threats of tomorrow".

"We will meet our obligations to our people to provide for the common defence, and we'll continue to do our part to support our allies in Europe and in NATO," he said.

- Trump 'contradicts himself'

Trump's criticism of NATO as "obsolete", his praise for Britain's decision to leave the European Union, and his softer approach towards Russia have unnerved Washington's allies.

But over the past week in Europe, his envoys have pressed the message that the United States is not retreating into isolation and remains committed to its global role.

They have also held to a hard line on maintaining sanctions against Moscow over Ukraine.

"The United States will continue to hold Russia accountable, even as we search for new common ground, which as you know, President Trump believes can be found," Pence said.

At NATO headquarters in Brussels on Thursday, Defence Secretary James Mattis said Russia must first "prove itself" and respect international law before there could be any improvement in relations strained by Moscow's Ukraine intervention and annexation of Crimea.

Mattis said the transatlantic bond was "as strong as I've ever seen it", and emphasised that America remained "rock solid" in its support of Article 5 -- NATO's core "one for all, all for one" collective defence tenet.

Likewise, US Secretary of State Rex Tillerson also indicated he would take a tough line in his dealings with Russia.

Following his first meeting with Russian Foreign Minister Sergei Lavrov in Bonn on Thursday, Tillerson said the US would cooperate with Moscow but only when doing so "will benefit the American people".

Explaining the apparent policy back-and-forth, Republican US Senator John McCain said Trump sometimes "contradicts himself, so some of us have learnt to watch what the president does as opposed to what he says".

"Make no mistake, these are dangerous times, but you should not count America out," he said in Munich on Friday.

While noting Pence's affirmation of US commitment to the transatlantic partnership, French Foreign Minister Jean-Marc Ayrault noted on Twitter that there was "not a word on the EU".

- 'Won't give up on Russia' -

Exasperated and worried by Trump's calling into question long-standing foreign policy assumptions, Europe's top politicians have warned Washington not to take transatlantic ties for granted.

They work both ways, they said, and benefit the United States as much as Europe.

Merkel on Saturday urged countries not to retreat from the international cooperation which she says is the only way to solve global problems.

"In a year in which we see unimaginable challenges we can either work together or retreat to our individual roles. I hope that we will find a common position," she said.

This includes working not only with Western partners, but also with Russia if possible and if Moscow once again respected the sovereignty and territorial integrity of other states such as Ukraine, she said.

She said it was "regrettable" that Europe had not managed to reach a stable relationship with Russia over the last 25 years.

"I will not give up on finding a way for better relations with Russia despite our different views on many questions," she said, hours before Lavrov, Russian foreign minister, was due to address the forum.