The West should be prepared for a "fully-fledged Russian invasion" of Ukraine that could include an attempted coup, cyber attacks and sabotage, NATO has warned.
Speaking at an event in Brussels, NATO Secretary General Jens Stoltenberg said on Friday that the build-up of Russian troops at the borders with Ukraine and Belarus is not necessarily the only way Moscow plans to put pressure on Western allies.
"We need to be prepared for a wide range of different forms of aggression," he said, listing several examples.
It comes after Russia launched a series of military drills near its borders with both of its former Soviet neighbours.
The Kremlin is thought to be mobilising infantry and artillery units in southwestern Russia, alongside war planes in Kaliningrad on the Baltic Sea and warships in the Black Sea and Arctic.
NATO has bolstered its presence in the Baltic Sea region in response, while the US has ordered a further 8,500 troops to be on high alert for deployment to eastern Europe.
Russia has repeatedly insisted it will not invade Ukraine, but foreign minister Sergei Lavrov warned on Friday: "There won't be a war as far as it depends on the Russian Federation, we don't want a war.
"But we won't let our interests be rudely trampled on and ignored."
Putin tells Macron Moscow's principal concerns not met
In a phone call with French President Emmanuel Macron on Friday, Vladimir Putin said that the US and NATO's proposals on security do not meet Moscow's main concerns.
But the Kremlin said Mr Putin would closely study the proposals before deciding on further action.
A build-up of as many as 150,000 Russian troops on the Ukrainian border began late last year as Russia demanded that Ukraine be banned from joining NATO and the alliance scales back its military operations in eastern Europe.
Moscow wants to keep its former Soviet neighbour in its 'sphere of influence' following its illegal annexation of Crimea in 2014.
Negotiations between Russia and the West last week proved largely unsuccessful, with US President Joe Biden telling Ukraine's president Volodymyr Zelenskiy in a phone call on Thursday that Russia could invade as soon as next month.
The US and NATO formally rejected Russia's demands this week but outlined a number of secondary areas where there could be room for negotiation and an opportunity to avoid full-scale war.
Russia faces devastating sanctions - including halt to gas pipeline
Meanwhile Washington and other Western allies warned the Kremlin it faces devastating economic sanctions if it decides to pursue military intervention.
US officials said Germany would not allow the newly-constructed Nord Stream 2 pipeline to bring gas from Russia if it goes to war.
And Whitehall sources have told Sky News that Britain is considering sending "hundreds more" troops to the Ukrainian border to bolster NATO's presence.
But despite the possibility of an imminent invasion, Ukrainian officials have tried to convey a sense of calm.
Its defence minister Oleksii Reznikov told parliament on Friday that the total number of Russian troops near Ukraine is comparable to Moscow's military build-up in the spring of 2021, when it eventually decided to pull its forces back.
"We haven't observed any events or actions of military character that significantly differ from what was going on last spring," with the exception of the deployment to Belarus, Reznikov said.