The Foreign Secretary says he is "gravely concerned" about reports of the Russian military gathering on the Ukraine border.
Dominic Raab says the UK will support Ukraine's sovereignty and "territorial integrity".
Ukraine's east continues to see clashes between Ukrainian forces and separatists backed by Russia.
Kremlin spokesman Dmitry Peskov has warned NATO not to send troops in response to the reported build-up of Russian soldiers, artillery and tanks near the border.
He says Russia would take extra measures if NATO were to take such action.
The warning comes as Moscow's leading diplomat said relations with the US and its allies have "hit the bottom".
As of yet, no date has been given for returning the Russian ambassador to Washington.
Russia recalled the representative after US President Joe Biden was asked in an interview if he thought Russian President Vladimir Putin was a "killer" and replied: "I do".
Russia's foreign minister Sergey Lavrov said Mr Biden's remarks are "appalling" and said Moscow had been forced to rethink ties with Washington.
Mr Biden has said the days of the US "rolling over" to Mr Putin are done.
Cover image: Library picture showing Moscow Kremlin's Spasskaya Tower (L), and St Basil's Cathedral (Picture: PA).