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.
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".
Meanwhile, Mr Peskov accused Ukrainian forces of staging "provocations" on the frontline in eastern Ukraine, where a fragile ceasefire is violated daily.
In the worst flare-up in recent months, four Ukrainian soldiers died in separatist shelling on 26 March near Shuma, a village in the Donetsk region. There have been only low-level incidents since then.
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.
BBC
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