Russia has amassed an estimated 130,000 troops along the border with Ukraine, putting Washington and Moscow into an increasingly tense Cold War-style stand-off.
Here is a timeline of the unfolding situation:
Troop movement
On 10 November, NATO warns Moscow about taking "aggressive action" after Washington reports unusual troop movements near the Ukrainian border.
On 28 November, Ukraine says Russia is massing nearly 92,000 troops for an offensive at the end of January or early February.
Moscow denies this and three days later accuses Kyiv of a military build-up of its own, demanding "legal guarantees" that it will never join NATO.
'Massive consequences'
On 7 December, US President Joe Biden threatens Russian counterpart Vladimir Putin with "strong economic and other measures" if he invades Ukraine.
On 16 December, the EU and NATO warn of "massive strategic consequences if there was a further attack on Ukraine's territorial integrity".
The next day Moscow puts forward proposals to limit US and NATO influence on former Soviet states.
Major cyber attack
On 14 January, a cyber attack briefly knocks out key government websites in Ukraine.
Kyiv says it has uncovered clues that Russia might be behind it.
Build-up in Belarus
On 17 January, Russian troops begin arriving in ex-Soviet Belarus for military drills, which Moscow says are aimed at "thwarting external aggression".
US officials say the size of the force is "beyond what we'd expect of a normal exercise".
Russian soldiers take part in a military drills in Siberia, Russia. Source: AAP/Russian Defense Ministry Press Service
Two days later, Washington announces an extra $200 million in security aid to Kyiv.
Biden fears 'invasion'
On 20 January, Mr Biden says any incursion of Russian troops is "an invasion" after appearing to suggest a "minor" attack on Ukraine might invite a lesser response.
Baltic states move in
On 21 January, ex-Soviet NATO members Estonia, Latvia and Lithuania say they will send anti-tank and anti-aircraft missiles to help Ukraine defend itself.
Russia demands the withdrawal of NATO troops from Romania and Bulgaria.
A thousand NATO soldiers and military equipment deployed in Romania. Source: Getty Images/Anadolu
Invasion fears grow
The next day, Britain claims Moscow is "looking to install a pro-Russian leader in Kyiv" and occupy Ukraine, which Russia dismisses as "disinformation".
Washington orders the families of its diplomats to leave the country, and later warns its citizens not to travel to Ukraine.
NATO on standby
On 24 January, NATO puts troops on standby for possible deployment and sends ships and fighter jets to bolster Europe's eastern defences.
The next day, Moscow launches exercises involving some 6,000 troops and at least 60 fighter jets in southern Russia near Ukraine and in Moscow-annexed Crimea.
On 26 January, Washington refuses to shut the NATO door on Ukraine and the alliance says many of Moscow's security demands are "unacceptable or unrealistic".
China warns the US
The US says it believes Mr Putin "is going to use military force between now and the middle of February".
The next day China warns that Russia's security concerns should be "taken seriously".
On 28 January, Mr Putin says the west has ignored "Russia's fundamental concerns" on NATO's expansion and deployed "strike weapons systems near Russia's borders".
UN showdown
On 31 January, Moscow accuses the US of whipping up "hysteria" as Washington says 30,000 Russian troops will be deployed in Belarus near the Ukrainian border by early February.
The next day Mr Putin says he hopes a solution can be found to the crisis but accuses Washington of using Kyiv as a "tool" against Russia.
'Psychological pressure'
On 2 February, the US sends 3,000 troops to fortify NATO forces in eastern Europe, which Russia calls a "destructive step".
Five days later, French President Emmanuel Macron shuttles between Moscow, Kyiv and Berlin in a bid to head off war, with the US saying Russia now has 110,000 troops on Ukraine's border with another 40,000 to arrive within a week.
Russia and Belarus begin 10 days of military manoeuvres on 10 February which Kyiv calls "psychological pressure".
Map showing positions of Russian troops around Ukraine. Source: SBS News
The head of NATO warns Moscow's military build-up has reached a "dangerous moment".
Russia says it withdraws some troops, another cyber attack
On 14 February Russia appears to open the door to a diplomatic solution. And on 15 February Moscow says some of its forces near the Ukrainian border were returning to their bases "having completed their tasks".
Kyiv claims its diplomatic efforts with Western allies have deterred an invasion.
However the head of NATO later disputed Russia's version of events, saying: "we have not seen any withdrawal of Russian forces."
On 16 February Ukraine's defence ministry and two banks are targeted in a cyber attack. Ukraine suspects Russia was behind it.
Kindergarten hit by pro-Russian separatists
On 17 February, tensions ramp up dramatically as pro-Russian separatists launch artillery fire on territory in the east of Ukraine, shelling a kindergarten and injuring three people.
Ukrainian President Volodymyr Zelensky says the attack "by pro-Russian forces is a big provocation".
Australia’s Defence Minister Peter Dutton warns Russia is on the cusp of an "all-out conflict" in Ukraine.
Joe Biden says he is 'convinced' Russia will invade Ukraine
On 18 February, Mr Biden says he is "convinced" that Mr Putin has decided to launch an invasion in Ukraine, saying he has "reason to believe" it will occur in "coming days" and will include an assault on the capital Kyiv.
Mr Biden says "the entire free world is united" against Russia.
On 19 February Mr Zelensky urges the West to better protect his country that he says has been acting as a "shield" against Moscow for eight years.
Meanwhile, Prime Minister Scott Morrison says Australia won't be sending any troops to Ukraine in the event of an invasion that he describes as "regrettably inevitable".
Joe Biden and Vladimir Putin agree to summit
On 21 February, French President Emmanuel Macron secures another urgent diplomatic meeting with Russia in the hope of dispelling any potential threat of invasion.
Mr Biden and Mr Putin agreed in principle to a summit over Ukraine, the office of French President Emmanuel Macron says, provided Russia does not invade its neighbour.
In a statement, the Elysee Palace says Mr Macron had pitched both leaders on a summit over "security and strategic stability in Europe".
Putin sends 'peacekeepers' to disputed Ukrainian territories
On 22 February, Mr Putin recognises the breakaway regions of Donetsk and Lugansk, in Ukraine's east, as separate states, and orders Russian troops to operate what he calls peace-keeping operations in the states.
Mr Putin makes an 90-minute long television address in which he describes Ukraine as an integral part of Russia's history.
US Secretary of State Anthony Blinken describes the move as 'shameful.'
European Union chief Ursula von der Leyen tweets Russia's move is a 'blatant violation of international law.'
Mr Biden says the US will impose sanctions preventing US citizens from engaging in new 'investment, trade and financing' in the regions.
The sanctions are separate to those being planned against Russia.
Britain's foreign minister Liz Truss said the UK would impose new sanctions on Russia.
Meanwhile, Australia has evacuated its diplomats from Ukraine as a safety measure, as Prime Minister Scott Morrison called on Russia to withdraw from the regions.
“It should unconditionally withdraw, back behind its own borders and stop threatening its neighbours,” Mr Morrison told reporters in Tasmania.
“It’s unacceptable, it's unprovoked, it's unwarranted.”
