Boris Johnson confirmed on Tuesday that the UK would contribute to Nato deployments if Russia invades Ukraine.
Claiming Moscow has a “gun to Ukraine’s head”, the Prime Minister warned Vladimir Putin that he would not hesitate to “toughen our national sanctions” following a virtual meeting on Monday night with other Western leaders, including Joe Biden, the US president.
Here we break down how the crisis reached this point and what could be next.
Up to 130,000 Russian troops, alongside 1,200 tanks, fighter jets and long-range missile batteries have massed on Ukraine’s eastern border. The troop build-up has sparked the biggest crisis in East-West ties since the Cold War.
Russia has put forward a list of security demands including a guarantee that Ukraine will never be allowed to join Nato and that alliance forces pull back in Eastern European countries that joined after 1997.
How Russia could invade Ukraine
Moscow has accused the US of ratcheting up tensions after Washington put several thousand troops on alert for possible deployment to boost Nato’s eastern borders.
A series of talks in various European cities this month have failed to ease tensions, though US Secretary of State Antony Blinken and Russian Foreign Minister Sergei Lavrov agreed at a meeting in Geneva on Friday to keep talking.
What’s behind the crisis?
There has been tension between Moscow and Kyiv since Ukraine declared itself independent of the Soviet Union in 1991.
The situation escalated in 2014 when Russia annexed the Crimean peninsula in southern Ukraine and sent troops to support a separatist uprising in the east of the country.
Russia-Ukraine crisis over the years
Ongoing clashes in Ukraine’s Donetsk and Luhansk regions have killed around 14,000 people and two million people have been displaced.
All attempts to reach a political settlement have failed so far.
A peace agreement signed in Minsk in 2015 stopped the worst of the fighting but has failed to deliver a settlement of the conflict because of disagreements between Kyiv and Moscow over how it should be implemented.
What does Putin want with Ukraine?
Mr Putin maintains that Ukraine is fundamentally part of Russian civilisation, both culturally and historically, and has questioned whether it is even a real country.
Mr Putin also sees Russian dominance of Ukraine as fundamental to Russian security. The wider context is that the crisis is a challenge to what he views as an unfair agreement imposed on Russia at the end of the Cold War.
He views Nato’s expansion towards Russia as an existential threat and claims Moscow’s military movements are a response to Ukraine’s growing ties to the alliance and hopes that via this crisis he can push back.
Asserting power over Ukraine is part of his push to affirm Russia’s place among world powers, including the US and China.
How big is the risk of invasion?
Russia has denied that it has any plans to invade Ukraine – but tensions are extremely high and Mr Putin has threatened “appropriate retaliatory military-technical measures” if “Western aggression” continues.
Mr Johnson said on Monday that “gloomy” intelligence suggested Russia was planning a lightning attack on Kyiv. Some British embassy staff and their families began leaving the Ukrainian capital this week.
The Prime Minister said: “The intelligence is very clear that there are 60 Russian battle groups on the borders of Ukraine, the plan for a lightning war that could take out Kyiv is one that everybody can see.”
A serviceman stands in a trench on the territory controlled by pro-Russian militants near frontline with Ukrainian government forces in the Luhansk region of eastern Ukraine Credit: Alexei Alexandrov /AP
The US agrees the chances are high. President Biden himself has said the military buildup looks like a threat.
“It’s clear Russia has no intention of de-escalating,” said John Kirby, a Pentagon spokesman, on Monday.
Not everybody agrees. Ukrainian officials have questioned this, saying they see no sign of attack in the next few weeks. The French government has issued similar assessments.
What happens next?
According to US officials, Russia is planning a false-flag attack as a pretext for war.
There could also be a plot to install a puppet government, MI6 has asserted – but Russia denies both claims.
Publicly, the United States and European allies have promised to hit Moscow financially like never before if Mr Putin does roll his military into Ukraine.
Some European Union members appear less willing to take severe action against Russia, which supplies about 40 percent of the bloc’s natural gas supplies.
Nato’s enhanced forward presence for Ukraine tensions
Lord Dannatt, the former head of the British Army, believes Ukrainians would “fight and fight hard”, and that would mean Russian fatalities and casualties.
Moscow still expects the US to come up with a written response to its security demands this week, Dmitry Peskov, Kremlin Press Secretary, said.
The US is looking at ways to mitigate the effects of a Russian gas blockade to try and supply allies in the eventuality that the stream is cut off.