Russia has said it would treat any move by the United States to provide the Ukrainian government with lethal weapons as a threat to its security.
Nato's expansion in Eastern Europe was another move causing deep concern in Moscow, Russian foreign ministry spokesman Alexander Lukashevich said on Thursday (5 February).
European leaders arrived in Kiev on Thursday to present a new peace initiative to end the conflict that has already cost at least 5,000 lives since April 2014.
American legislators last year passed an act that could make it easier for the US to supply Ukraine's government with lethal aid.
"Taking into account the revanchist plans of the 'party of war' in Kiev, this is not only fraught with the escalation of the situation in the south-east [of Ukraine] but threatens the security of the Russian Federation," Lukashevich said, as quoted by Reuters news agency.
Nato defence ministers signed off on a plan to bolster the organisation's presence in Eastern Europe on Thursday.
The move is a response to Russia's increased militarism in the region, after it annexed the Black Sea peninsula of Crimea from Ukraine in March 2014.
The alliance will establish a network of command centres in Eastern Europe, meant to rapidly reinforce the region in the case of any threat from Russia.
It also agreed on establishing two new regional headquarters and boosting the size of the regional rapid reaction force.
Nato boss Jens Stoltenberg said the measures amounted to "the biggest reinforcement of our collective defence" since the end of the Cold War. The UK will provide 1,000 troops and four fighter jets to help boost Nato's clout in the Baltic states of Latvia, Estonia and Lithuania, which do not have their own air forces.
While Nato considers its actions to be defensive in the face of increased Russian assertion, Russia would see the move in a fundamentally different light.