Russia 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.
Mr Peskov accused Ukrainian forces of staging “provocations” on the frontline in eastern Ukraine, where a fragile ceasefire is violated daily.
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.
A Nato official told Reuters news agency that Russia was undermining efforts to reduce tensions in eastern Ukraine and Nato ambassadors had met on Thursday to discuss the situation.
“Allies share their concerns about Russia’s recent large-scale military activities in and around Ukraine,” the official said.
Ukraine’s President Volodymyr Zelensky joined the criticism, saying “military exercises and possible provocations along the border are traditional Russian games”.
In a phone call with Mr Zelensky on Friday, US President Joe Biden “affirmed the United States’ unwavering support for Ukraine’s sovereignty and territorial integrity in the face of Russia’s ongoing aggression in the Donbas and Crimea”, the White House said in a statement.
Last month Russia conducted military exercises in Crimea, the Black Sea peninsula it annexed from Ukraine in March 2014.
The rebels in the Donbas region of eastern Ukraine also rely on Russian help. Western governments say Russia has deployed regular troops there, along with heavy weapons. That is denied by the Kremlin, which says Russian “volunteers” are helping the rebels, who seized a swathe of the Donetsk and Luhansk regions in April 2014.
Ukraine’s army commander Gen Ruslan Khomchak says Russia has deployed 28 battalion tactical groups near Ukraine’s eastern border and in Crimea, which would amount to 20,000-25,000 troops. Russian officials have not confirmed that, nor given any precise figures.
According to Gen Khomchak, Russia also has nearly 3,000 officers and military instructors in the rebel units in eastern Ukraine.
“The Russian Federation moves its armed forces within its territory at its discretion,” Kremlin spokesman Mr Peskov said. He added that “it should not worry anyone and does not pose a threat to anyone”.
The Russian military has confirmed that an airborne assault brigade, about 4,000 troops is being redeployed to Crimea from Volgograd in southern Russia this year.
Since 2014 Russia has considerably beefed up its military presence in Crimea, including its key Sevastopol naval base.
— Based Finland 🇫🇮 (@Based_Finland) April 1, 2021