Nato has seen no sign that Russia is slowing its troop build-up on its border with Ukraine, the head of the military alliance said.
Jens Stoltenberg said that the massing of soldiers and military hardware by Moscow was provocative and undermined the security of Europe.
The Nato chief, speaking alongside Ukrainian President Volodymyr Zelenskyy, urged Russia to de-escalate and to respect Ukraine’s territorial integrity.
“We have stepped up our reconnaissance and intelligence collection, and we will continue to monitor the situation very closely.”
The US says Russia has amassed more than 100,000 troops on Ukraine's borders, possibly in preparation for an invasion. Moscow says its actions are purely defensive and accuses Kiev and the West of provocative behaviour.
The Kremlin said on Thursday it was ready to send a government negotiator “at any moment” to start talks with the US on the security guarantees it is seeking to defuse the crisis.
Meanwhile, EU leaders met in Brussels for the European Council, with officials from the Baltic and former Soviet states particularly vocal about the threat they say Russia poses.
Western powers have repeatedly threatened to impose further sanctions on Moscow if it takes any more aggressive steps.
“We are probably facing the most dangerous situation in the last 30 years, I am talking about not only Ukraine but the eastern flank of Nato,” said Lithuanian President Gitanas Nauseda.
Referring to sanctions, Mr Nauseda said the EU has “enough tools to stop Russia from its aggressive behaviour”.
“We really are facing a series of attacks. I see them all as associated,” Latvia Prime Minister Krisjanis Karins said, listing Middle Eastern migrants sent by Belarus to EU borders, artificially high natural gas prices orchestrated by Moscow and Russian disinformation.
Since 2014, the EU has imposed a series of sanctions on Moscow after its annexation of the Crimea.
Slovenia's Prime Minister Janez Jansa, whose country holds the rotating EU presidency, said that “there is no doubt that Russia is using military power to make pressure”.