UK Prime Minister Boris Johnson was the latest NATO leader to try and show a united front and stand against perceived Russian aggression. 

Visiting Brussels, where the alliance is headquartered, Johnson made it very clear that hostilities would not be tolerated while reiterating that NATO is seeking a diplomatic solution to the crisis.

But treading the line between averting war and not giving in to Russian demands is a delicate balancing act. Russia is unhappy NATO is considering making Ukraine a full member of its military alliance. Moscow says that any NATO eastern expansion is a sign of aggression. NATO disagrees, saying sovereign nations have the right to apply to join the alliance.

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In a news conference after meeting NATO Secretary-General Jens Stoltenberg, Johnson said:

"If we can keep a strong grip on the fundamental principles that define our alliance, and combine strong deterrence with patient diplomacy, then we can find a way through this crisis. But the stakes are very high, and this is a very dangerous moment."

Just how precarious a situation Europe finds itself in was highlighted by Stoltenberg. 

"This is a dangerous moment for European security. The number of Russian forces is going up, the warning time for a possible attack is going down. NATO is not a threat to Russia. But we must be prepared for the worst while remaining strongly committed to finding a political solution."

After leaving Brussels, Johnson made his way to Poland, another key NATO member with a large Ukrainian border.

He met senior Polish politicians and some of the 1,000 UK troops stationed there, announcing that an additional 350 soldiers are on standby to join them.