Russia's intentions are far from clear, but here's a quick guide to the latest developments.
Is Russia preparing to invade Ukraine?
Multiple sources have reported large Russian military movements towards the eastern Ukraine border and into Crimea, which Russian forces annexed from Ukraine in March 2014. Many of these reports have appeared on Twitter, such as tweets by Jane's intelligence information group about Iskander short-range missiles.
The Kremlin has not given details. President Vladimir Putin's spokesman Dmitry Peskov said moving troops across Russian territory was an "internal affair". Some of the troops, including units in Crimea, have been on exercises. But Mr Peskov also accused Ukraine of staging "provocations".
Ukrainian intelligence sources told the BBC that the extra forces amount to 16 battalion tactical groups, which would be up to 14,000 soldiers. In total, according to the Ukrainian presidency, Russia now has about 40,000 on the eastern border and about 40,000 in Crimea.
On 13 April Russian Defence Minister Sergei Shoigu accused Nato of "threatening" actions and said Russia had responded by sending two armies and three formations of airborne troops to its western borders, to conduct exercises. He gave no detailed figures or locations.
So is this an invasion force? It could be, but analysts say a big invasion is unlikely. Infiltration would be more Russia's tried-and-trusted method. Russian special forces without insignia - nicknamed "little green men" - took over Crimea in 2014.
Nato Secretary General Jens Stoltenberg called the new Russian build-up "unjustified and deeply concerning" and said it was "the largest massing of Russian troops since the illegal annexation of Crimea".
Ukraine, Nato and Western governments have also long accused Russia of deploying regular units and heavy weapons in separatist-held eastern Ukraine. The Kremlin denies that and calls any Russian troops there "volunteers".
Speaking at Nato HQ in Brussels, Ukrainian Foreign Minister Dmytro Kuleba said Russia was "openly threatening Ukraine with war and destruction of our statehood". But unlike in 2014, he added, "Russia won't be able to catch anyone by surprise anymore".
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