Russian President Vladimir Putin says clashes between Russia and Ukrainian forces are inevitable, it is just a question of time.
Ukraine's President Volodymyr Zelenskyy has declared martial law and urged his citizens to not panic.
What's the latest?
The Ukrainian president said Russia has carried out missile strikes on Ukrainian infrastructure and border guards and that explosions had been heard in many cities.
Earlier, in a late-night television appearance, Putin said he has authorised a special military operation in Ukraine's Donbas region, which includes the Luhansk and Donetsk regions.
Putin said clashes between Russian and Ukrainian forces were inevitable, that it was only a matter of time.
The scope of this special military operation isn't clear but Putin has called on Ukrainians to lay down their arms. He warned that any external intervention will be met with a swift response.
Before Putin's announcement, Zelenskyy gave an emotional address, speaking in Russian and appealing directly to the Russian people:
How did we get here?
- For weeks, Russia has been massing about 150,000 troops along Ukraine's border to the north, east and south.
- Western leaders have said any movement by Russia's military over Ukraine's border would draw a strong response.
- Earlier this week, Putin ordered Russian troops into Ukraine's eastern Luhansk and Donetsk regions, parts of which are under separatist control.
- There are fears Russia will push its troops further into Ukrainian territory.
- Ukraine will wait and see what happens next. For the meantime, the airspace over all of Ukraine has been shut down to civilian air traffic.
What's the latest on where the troops are?
Troops, equipment, armoured vehicles, aircraft and missiles have been gathering in several positions along the border.
Those pressure points sit on three sides, from the north in Belarus, the south in Russian-annexed Crimea, and of immediate concern, the rebel-held eastern areas of Donetsk and Luhansk.
What do the latest satellite images show?
This map shows Russia's military build-up since January, as confirmed by satellite images.
The latest images released by space tech company Maxar Technologies show Russian troops, housing, military equipment, helicopters, jets, armoured vehicles and long-range missile defence systems.
What are leaders saying?
Putin's declaration of a "special military operation" came as the UN Security Council was in an emergency meeting on the crisis.
In response, Ukraine's ambassador to the Security Council Sergiy Kyslytsya said that Putin has "declared war on Ukraine".
Ukraine Foreign Minister Dmytro Kuleba says a "full-scale" war has been launched by Russia.
As for the next steps, US Pentagon press secretary John Kirby said Russian forces were in an advanced state of readiness.
ABC/wires
