Andrei Kelin, Russia’s ambassador to the UK, warned there is “a pretty serious risk” of conflict. Ukrainian President Volodymyr Zelensky has claimed Moscow is planning to overthrow his government on Wednesday.
Nato warned Russia that using force against Ukraine would “come at a cost”.
British troops have been ordered to Germany, ready to respond to Russian threats.
Mr Kelin said: “There is a risk of war on our border.
“It’s pretty serious. There is a possibility that there is a spark and an incident can occur.
“Nato is stepping up its presence along the borders of the Russian federation. We have lots of manoeuvring now in the Baltic Sea.
“We have strategic aviation, with nuclear warheads. It’s dangerous.”
Russian forces have been building up near Ukraine’s eastern Donbas region, with tanks heading to the area along with an estimated 92,000 troops around the border.
But Moscow has dismissed claims it is preparing for action as “alarmist”.
Experts believe Russian President Vladimir Putin is sending a message to the West that it will not tolerate any attempt to take Ukraine into Nato.
Mr Kelin said the movement of British tanks closer to Russia was fuelling military “tension”.
He insisted it was “absolutely not” the case that Moscow is preparing to invade Ukraine.