Russia has warned Nato against sending any troops to help Ukraine, amid reports of a large Russian military build-up on its borders.
Kremlin spokesman Dmitry Peskov said Russia would take "additional measures" if Nato were to make such a move.
Sporadic, low-level clashes continue in eastern Ukraine between Ukrainian troops and Russian-backed separatists.
US forces in Europe are now on higher alert, citing "escalations of Russian aggression" in the area.
If you thought a global pandemic was dangerous enough, think again.
The Russians say that their forces amassing on Ukraine's border are just there for "exercises". Do we believe them? Of course not and besides, the Russians are moving tanks into Crimea and moving other military equipment into Eastern Ukraine. The Russians intend to continue what they started back in 2014, to take the whole country.
Biden has already spoken to the Ukrainian president and offered "unwavering support".
Are we about to go to war against Russia?