"The risk of conflict is real. Russia's aggressive actions seriously undermine the security order in Europe," Stoltenberg said after a video meeting of NATO foreign ministers.

"The Russian military build-up has not stopped, it continues and they are gradually building up with more forces, more capabilities."

Tensions have soared in recent months as the West has accused Moscow of threatening a full-scale invasion of its neighbour by massing some 100,000 troops on its border.

High-ranking officials from the United States and Russia are set to hold talks in Geneva on Monday, after Moscow laid down a list of demands for Washington and the Western military alliance.

Those will be followed Wednesday by a meeting between Russia and all 30 NATO members -- the first such encounter since July 2019.

"NATO will engage in dialogue with Russia in good faith and on substance but we must also be prepared for the possibility that diplomacy will fail," Stoltenberg said.

"Today ministers stressed that any further aggression against Ukraine would have significant consequences and carry a heavy price for Russia."

The West is eyeing tough financial sanctions on Moscow if it stages an invasion, but NATO is not ready to get involved militarily to protect Ukraine.

- 'Core principles' -

The Kremlin shocked the United States and its allies by publishing two proposed treaties late last year calling for NATO to rule out future membership for Ukraine and to roll back its forces near Russia's border.

The West has rejected what it calls a bid by Moscow to dictate the future of independent partners.

"All our allies sent a very clear message that we will not compromise on core principles, including the right for every nation to decide its own path," Stoltenberg said.

But diplomats said there were key differences among European allies over the approach NATO should take at the talks with Russia next week.

Some members insist the focus has to remain on getting Russian President Vladimir Putin to back off from Ukraine and that Moscow's proposals need to be rejected out of hand.

Others have said the meeting could already help to kickstart a negotiation process and there are issues where there could be room for discussion.

Jittery European allies pushed for a clearer picture from the United States of its game plan for the talks in Geneva, with EU head Ursula von der Leyen insisting "Europe has to be involved".

Stoltenberg said that the US had "been very clear that there will be no decisions about European security, no discussions about European security without the Europeans at the table".

Moscow's demands have also frayed nerves in non-NATO European allies Finland and Sweden, which have rejected claims anyone should have a say over the decision to join the alliance.

NATO Secretary General Jens Stoltenberg spoke to the leaders of the two nations Friday to insist the alliance "respects the right of every country to choose its own path".