NATO's Secretary General has criticised the build-up of Russian forces near Ukraine as "unjustified, unexplained and deeply concerning".
Jens Stoltenberg was speaking after a meeting with Ukraine's Foreign Minister, Dmytro Kuleba, when he said Russia must "stop its provocations and de-escalate immediately".
Speaking at NATO's Brussels headquarters, Mr Stoltenberg also warned Russia against interfering with a process which could see Ukraine become a member of the military alliance.
The Russian build-up is thought to be the biggest since the country annexed Crimea in 2014.
But Russia's defence minister blamed a perceived military threat from NATO for the country's deployment of two armies and three airborne formations to western regions.
Speaking at a meeting with the top military brass, Sergei Shoigu said the manoeuvres would last for another two weeks "as a response to the alliance's military activities threatening Russia".
The Kremlin has also warned that Russia will take extra measures if NATO sends troops to Ukraine in response.
NATO already has troops, including British personnel on Operation Orbital, in Ukraine for training and joint exercises.
Referring to the annexation of Crimea, Mr Kuleba said Russia "will not be able to catch anyone by surprise anymore".
"Should Moscow take any reckless move or start a new spiral of violence, it will be costly in all senses," he added.
The White House said US President Joe Biden voiced concern over the Russian build-up and "called on Russia to de-escalate tensions," during a phone call on Tuesday with Russian President Vladimir Putin.