The US and its allies have warned Russia is ready to invade Ukraine at any moment - but where has the dispute come from and could it mean all-out war?
At the end of last year, Ukrainian and US officials began sounding alarms about a build-up of Russian troops along Ukraine's border.
The Kremlin continuously denies it is planning an invasion of its neighbour and has called the West's recent response as "hysteria".
No-one really knows what Russia's aim is but it claims it is trying to bring NATO to the negotiating table to settle some of its long-held grudges against the alliance, particularly the possibility of Ukrainian membership.
NATO members have also been supplying Ukraine with anti-air and anti-tank weapons to help the former Soviet state to "defend" itself.
With more than 150,000 Russian troops now on the border, the US has warned an invasion could be imminent and many western nations have urged their citizens to leave Ukraine "immediately".
But where has this threat of aggression come from and why is Ukraine again at the forefront?
What's the background?
Ukraine has long been a source of tension between the West and Russia, which has competed for it to be part of their spheres of influence.
It was a major theatre of both the World Wars as the Germans sought to wrestle control of it from the Russians, but ultimately it remained in the hands of the Soviet Union until it gained independence in 1991.
Since then, being either Western or Russian focused, has played a huge part in internal Ukrainian politics.
In 2013, pro-Russian president Viktor Yanukovych backed away from a friendship agreement with the European Union in favour of closer ties to Russia.
Anti-government protests by the Western supporting population in the capital of Kyiv broke out, and riots in January 2014 left almost 100 people dead.
At the same time, pro-government protests erupted in the eastern parts of Ukraine that have a large population of ethnic Russians.
Mr Yanukovych fled Ukraine in February 2014 and a pro-European government was elected.
The ousting of the president caused Russia's President Putin to invade and annex the Crimean peninsula as well as encouraging an armed uprising in the east of the country.
The annexation of Crimea was met with fury by the West and led to Russia being expelled from the G7, starting a new period of frosty relations between the two.
Pro-Russian militias also launched a separatist movement in eastern Ukraine, leading to a bitter conflict that was paused after a peace agreement brokered by France and Germany was signed in 2015.
The agreement was a diplomatic coup for Moscow, requiring Ukraine to grant broad autonomy to the rebel regions and offer a sweeping amnesty to the rebels.
What's been happening recently?
In December, the US said it believed Russia is planning to deploy 175,000 soldiers along the border, with almost 100,000 troops already there.
By February the US said Russia now had at least 150,000 troops - with reports of 170,000 - in position and was ready to invade at any time.
Ukraine has also said the pro-Russian militias in the east have been breaking the 2015 ceasefire agreement more frequently.
President Putin had claimed Russian troops were beginning to withdraw from the border - but there has been widespread doubt over his assertions and reports suggest forces have actually been moving towards Ukraine.
In an attempt to diffuse the situation, leaders and senior diplomats from the UK, Germany and France have been flying to Kyiv and Moscow to try and find a compromise. So far, little progress has been achieved to de-escalate tensions.
After meeting British foreign minister Liz Truss, Russian foreign minister Sergei Lavrov accused her of being "deaf".
The most recent call between Presidents Biden and Putin also produced "no fundamental change" to the threat of imminent war.
President Putin earlier complained in a call with French President Emmanuel Macron that the US and NATO have not responded satisfactorily to his demands that Ukraine be prohibited from joining the military alliance and that NATO pull back forces from Eastern Europe.
The UK has sent troops to Poland and said it is prepared to deploy more to eastern European NATO nations, including the Baltic States, if Russia does invade.
The UK has also been supplying advanced weaponry to Ukraine, citing the 1994 Budapest Memorandum which committed the US, UK and Russia "to respect the independence and sovereignty and the existing borders of Ukraine.”
Russia has reacted with anger to the British armaments arriving in Ukraine despite also being signatories of the agreement.The UK also took the unusual step of publicly accusing the Kremlin of preparing a former Ukrainian MP to take over the country after Russia toppled the current government.
The foreign office said they believe Yevhen Murayev is being prepared as a new leader.
He has reportedly spoken out in the past in support of Russia’s annexation of Crimea from Ukraine in 2014.
Over in the US, the Americans have been saying with near certainty they expect Russia to invade imminently and have been warning of a "false flag" by the Russians to justify an invasion.
A false flag operation is when one country fabricates an aggressive action by an opposing nation in order to justify an invasion.
Indeed there are fears the shelling of a nursery on February 17 in government-controlled Ukraine could be used a pretext by Russia, even though the evidence suggests fire came from the separatists.
Russian-backed rebels and Ukrainian forces traded accusations that both sides had fired across the eastern Ukraine ceasefire line
President Biden has been keen to emphasise the US' support of Ukraine and commitment to Europe following Donald Trump's isolationist rhetoric during his term in office, which led to alarm across NATO. President Biden also said on January 20 he believed President Putin is "likely to move in… he has to do something.”
Due to the US warnings, many nations including the UK have told their citizens to leave Ukraine and have said they cannot guarantee flights out of the country if Russia does invade.
Russia has also accused the west of "hysteria" and denied it has any plans to invade.
Could NATO troops defend Ukraine?
Although Ukraine has a series of agreements with NATO it is not a member of the alliance, so it is under no obligation to defend it if it is attacked.
President Biden and NATO General Secretary Jens Stoltenberg have ruled out sending soldiers to fight Russian forces in Ukraine, but have not ruled out other forms of military intervention.
What does Russia want?
The Kremlin has accused Ukraine of failing to honour the 2015 peace deal and criticised the West for failing to encourage Ukrainian compliance. Ukraine, in turn, has pointed to ceasefire violations and insists there is a continuing Russian troop presence in the rebel east despite the Kremlin’s denials.
The dispute is also part of the wider geo-political tensions between NATO and Russia.
Russia has long accused NATO of encroaching on its borders, something it describes as unacceptable.
Moscow has strongly criticised the US and its NATO allies for providing Ukraine with weapons and holding joint drills, saying this encourages Ukrainian hawks to try to regain the rebel-held areas by force.
Mr Putin has repeatedly described Russians and Ukrainians as “one people” and claims Ukraine unfairly received Russian lands when the Soviet Union broke up.
Mr Putin has also strongly emphasised that Ukraine’s aspirations to join NATO represent a red line for Moscow.
At a press conference held at the end of last year, he said: "We have made it clear that NATO’s move to the east is unacceptable."
The UK Defence Secretary Ben Wallace, in an essay on the situation in Ukraine, said Russia's demands were based on false information and were designed to distract the West and justify the Kremlin's goals.
Mr Wallace said: "Only five of the 30 allies neighbour Russia, with just 1/16th of its borders abutted by NATO."
He pointed to the nations joining voluntarily arguing there had been no deliberate move to the east to threaten Russia.Mr Wallace also criticised Mr Putin's claims over Ukraine and what is in the best interest of Ukraine for not involving the Ukrainian people.
Finally, he noted Russia has several times since the fall of the Soviet Union recognised the sovereignty of Ukraine and agreed to their borders.