Boris Johnson has warned the crisis in Ukraine is reaching its "most dangerous moment" - with intelligence showing something "absolutely disastrous" could happen very soon.
The Prime Minister urged Russia to step back from the brink at a press conference in Brussels with NATO Secretary-General Jens Stoltenberg.
Mr Johnson flew out to Brussels for crunch talks over the mounting threat from Russia, before travelling to Poland to meet President Mateusz Morawiecki.
Asked about the threat of war, Mr Johnson said: "I honestly don't think a decision has yet been taken (by Moscow). But that doesn't mean that it is impossible that something absolutely disastrous could happen very soon indeed.
"And our intelligence, I'm afraid to say remains grim. We're seeing the massing of huge numbers of tactical battalion groups on the border with Ukraine."
"This is probably the most dangerous moment, I would say, in the course of the next few days, in what is the biggest security crisis that Europe has faced for decades, and we've got to get it right.
"And I think that the combination of sanctions and military resolve, plus diplomacy are what is in order."
He warned Russia that Europe must remain "whole and free" and it was "non-negotiable" to allow Ukraine to aspire to join NATO.
Mr Johnson did not rule out further military support for Ukraine in the event of an invasion by Russia.
He said: "We will consider what more we can conceivably offer. The Ukrainians are well prepared, there are things we've offered that they in fact don't seem to need because they think they have them in enough numbers already.
"It's possible, I don't want to rule this out, but at the moment we think the package is the right one.
"But I want to stress it would be an absolute disaster if it was to come to that and if there was to be serious bloodshed on Ukrainian soil."
Mr Stoltenberg warned Russian President Vladimir Putin that confrontation would have consequences.
"Russia has a choice: they can either choose a diplomatic solution - and we're ready to sit down - but if they choose confrontation, they will pay a higher price," he said.
"There will be economic sanctions. There will be an increased NATO military presence in the eastern part of the alliance, and the UK is really an important part of that."
He said he had written to Russian foreign minister Sergei Lavrov to offer more talks.
Foreign Secretary Liz Truss travelled to Moscow on Thursday to meet Mr Lavrov, where she warned aggression was "seriously undermining Russia's international standing."
At a joint appearance with Mr Lavrov, Ms Truss accused Russia of "attempting to destabilise Ukrainian democracy" through "hybrid warfare and amassing over 100,000 troops on the border in a threatening manner".
"These acts have actually had the effect of strengthening Nato's resolve and turning the Ukrainian people further away from Russia," she said.
"There are also serious implications for energy supplies at a time of rising gas prices."
She warned that a Russian invasion would result in a "prolonged and drawn-out conflict".
The UK would put in place "severe sanctions targeting individuals and institutions".
