Russia has amassed its troops around Ukraine's border in what NATO chiefs have called the biggest military build-up since the Cold War.
It is estimated 150,000 troops have gathered on the northern, eastern and southern borders of Ukraine.
Russian troops have already moved into Ukraine's eastern regions Donetsk and Luhansk after Russian President Vladimir Putin recognised them as independent.
Here's what satellite images show us about where those deployments are.
In the north
Belarus sits on Ukraine's northern border.
Belarusian President Alexander Lukashenko, who has had the political and financial backing of Russia, has said it is "clear whose side Belarus will be on" if war erupts.
Russia's deployment of troops to Belarus earlier this year raised concerns in the West that Moscow could stage an attack on Ukraine from the north.
Russia said its military presence was for scheduled military exercises. But those drills, which were meant to end on February 20, have been extended.
Satellite images taken by space tech company Maxar Technologies show missiles, armoured equipment and troops accumulating at Luninets Air Base, Mazyr, Osipovichi, Rechitsa and Zyabrovka.
Let's take a closer look at those images and their time line.
On January 30, satellite images showed SS26 Iskander missiles in the Osipovichi training area in Belarus.
Iskander missiles are wheeled, mobile-launched, short-range ballistic missiles that have a range of up to 500 kilometres. Kyiv is about 430km from the Osipovichi training area.
A few days later, on February 4, Sukhoi Su-25 ground attack aircraft were seen at Luninets airfield in Belarus, not far from the Ukrainian border.
The Sukhoi Su-25 aircraft are designed to target tanks, armoured vehicles and other ground targets.
By February 18, dozens of Su-25 ground attack aircraft could be seen at Luninets, along with helicopters, an S-400 air defence unit, ground force equipment and a drone unit.
An S-400 is a long-range surface-to-air missile defence system that can strike aircraft and cruise missiles within a 400-kilometre range.
On February 4, satellite images showed V D Bolshoy Bokov airfield near Mazyr before a deployment. By February 22, there was a new deployment of assembled vehicles.
On February 4, battle groups and artillery deployments, as well as troop housing, could be seen further to the east, in Rechitsa, Belarus.
By February 15, satellite images showed attack helicopters deployed at Zyabrovka airfield in Belarus.
And on February 16, the same day Russia announced it would withdraw troops from Belarus after joint military drills, troops and equipment could be seen in the Brestsky training area.
Russia and Belarus are united against the growing NATO military alliance.
Putin claims that after the Cold War, NATO promised it would not expand eastward to former Soviet states. But that didn't happen and over the decades several countries including Estonia, Latvia, Lithuania, Bulgaria and Hungary joined the bloc.
Ukraine has repeatedly said it also wants to join NATO and, in 2008, NATO promised eventual membership.
Putin wants NATO's guarantee that it will not ever accept Ukraine into the fold.
Since Ukraine won independence from the Soviet Union, it has tried to forge closer ties with the West. But it's an allegiance that makes Russia nervous because it doesn't want NATO on its border.
To the south
Russian troops, equipment and aircraft have also accumulated on Ukraine's southern border in Crimea, which was annexed by Russia in 2014.
At the time, Ukraine's Kremlin-leaning president Viktor Yanukovych had been ousted after he rejected an association agreement with the European Union in favour of closer ties with Moscow.
Russia responded by annexing Ukraine's Crimean Peninsula and threw its weight behind a separatist insurgency that broke out in Ukraine's east.
Taking control of Crimea gave Russia access to the Sevastopol naval base, which is the headquarters of Russia's Black Sea Fleet. This allows Russia to wield more power in the Black Sea and largely block Ukraine off.
On February 10, helicopters and troops could be seen at Lake Donuzlav, which has immediate access to the Black Sea. By February 15, military equipment was positioned in convoy.
Satellite images also showed a tent camp and equipment at Oktyabrskoye airfield, north of Simferopol.
And troops could also be seen gathered further south at Bakhchysarai.
To the east
Ukraine's eastern border is where things are now really heating up.
Putin has officially recognised the independence of the Moscow-backed rebel regions in eastern Ukraine and so-called "peacekeeping forces" have been sent in.
Here's what the satellite images from Kursk, Belgorod, Yelnya, Millerovo, Primorsko Akhtarsk Airbase and Soloti can tell us about where some of those Russian troops are based.
In Kursk, about 110 kilometres to the east of Ukraine's border, there are tents and troop housing, as pictured on February 9.
On February 13 in Belgorod, less than 20 kilometres to the north-west of the border with Ukraine, Russian helicopter units were pictured. By February 22, several new deployments of troops and equipment had been established in the area.
On February 13, armour and artillery could be seen loaded on flat cars in a railyard in Yelnya, which sits about 300 kilometres north of the border.
The same day, to the south-east, a new Su-34 fighter deployment could be seen at Primorsko Akhtarsk Airbase.
In Soloti, about 25 kilometres from Ukraine's border, a battle group in formation could be seen.
And more recently, on February 18, more Su-25 aircraft deployments were at Millerovo Airfield, which borders on Ukraine's Luhansk region.
For weeks, Western governments have warned that any movement of military forces across the Ukrainian border would draw a strong response.
Still, Putin has authorised his troops to cross Ukraine's border into rebel-held regions.
US President Joe Biden declared the move "the beginning of a Russian invasion of Ukraine".
NATO Secretary General Jens Stoltenberg said that, if Russian troops push further into Ukraine, the West would move in lock step.
"If Russia decides, once again, to use force against Ukraine, there will be even stronger sanctions, even a higher price to pay," Mr Stoltenberg said.
Biden has announced new sanctions on Russian banks.
"[Russia] can no longer raise money from the West and cannot trade its new debt on our markets, or European markets either," Biden said.
"We'll also impose sanctions on Russia's elites and their family members."
ABC/wires
