Russia is to alter its military doctrine as a result of the Ukraine crisis and Nato's presence in eastern Europe, a top Russian official says.
Mikhail Popov, a Kremlin adviser, said that deteriorating relations with the US and Nato would be reflected in the updated military strategy.
Nato said on Monday it would boost its presence in eastern Europe to protect its members.
Ukrainian troops are battling pro-Russian rebels in east Ukraine.
Ukraine's defence minister on Monday accused Russia of launching a "great war" that could claim tens of thousands of lives - claims dismissed by Russia, which denies actively supporting the rebels.
Popov, deputy secretary of Russia's National Security Council, told Russia's RIA news agency that "the military infrastructure of Nato member states" was "getting closer to [Russian] borders, including via enlargement".
Nato's actions were one of the key "external threats" to Russia, he said.
"Nato's planned action... is evidence of the desire of US and Nato leaders to continue their policy of aggravating tensions with Russia", Popov said.
There were no details on how the doctrine might change.
Nato announced its plans on Monday for a rapid response force of several thousand troops to protect eastern European members against possible Russian aggression.
The force, to be made up of troops provided by member states on a rotating basis, would be able to be deployed within 48 hours, Nato secretary general Anders Fogh Rasmussen said.
Military equipment and supplies would be pre-positioned in member states in the east so the force could "travel light, but strike hard if needed", he added.
Rasmussen insisted that the plans would not breach the 1997 Nato-Russia Founding Act, which forbids the presence of permanent bases in eastern and central Europe.
The new measures are set to be approved at a Nato summit in Wales this week.
The Nato security alliance covers 28 member states, including eastern European countries such as Poland and the Czech Republic. It does not include Ukraine.
Ukraine's Prime Minister Arseny Yatsenyuk has said that he aims to put the country on the path towards Nato membership.
Russian Foreign Minister Sergei Lavrov on Tuesday said such efforts were "undermining" attempts to reach a peace deal with the rebels.
Crisis talks between Ukraine officials, rebels and Russian envoys ended without agreement on Monday.
Ukraine's army has been forced to retreat amid a series of gains by pro-Russian rebels in both the Luhansk and Donetsk regions, and further south around the port of Mariupol.
On Monday, Ukraine's army said it had been forced to withdraw from Luhansk airport after it was attacked by Russian tanks.
The acting Luhansk region administration chief Irina Verihina told Ukraine's 112 TV: "Our troops have withdrawn, but the runway is completely destroyed. There's no way planes can land there."
The UN's refugee agency estimates that at least 260,000 people have been displaced inside the country, with most of those affected from eastern Ukraine.
'I CAN TAKE KIEV'
Ukrainian President Petro Poroshenko has accused Russia of "direct, overt aggression against Ukraine".
Russia has repeatedly denied Ukrainian and Western accusations that it is providing troops and equipment to the rebels.
Meanwhile, a Russian official responded to allegations that Russian President Vladimir Putin had commented: "If I want to, I can take Kiev in two weeks".
The reported comments were said to be made in a phone call to European Commission President Jose Manuel Barroso, and were reported in Italy's La Repubblica newspaper.
Kremlin adviser Yuri Ushakov said that whether or not the words were spoken, the quote "was taken out of context and had a totally different meaning", Russia's Itar-Tass news agency reported.