Russian President Vladimir Putin has said his country will move missiles closer to Europe over "concern" about Nato expansion.
Moscow will deploy S-400 surface-to-air missiles and nuclear-capable Iskander missiles in the exclave of Kaliningrad, a pro-Kremlin lawmaker told the state-owned RIA news agency.
"Why are we reacting to Nato expansion so emotionally? We are concerned by Nato's decision making," Mr Putin said in an interview due to be broadcast later on Monday.
"What should we do? We have, therefore, to take countermeasures, which means to target with our missile systems the facilities, that, in our opinion, start posing a threat to us," he added.
Speaking to a Nato parliamentary meeting in Istanbul, Nato's Secretary General Jens Stoltenberg said: "Everything Nato does is defensive, proportionate and fully in line with our international commitments.
"Before Russia's aggressive actions in Ukraine, Nato had no plans to send troops to the Eastern part of our alliance."
He added: "Nato's aim is to prevent a conflict, not to provoke a conflict. Moreover, we firmly believe and we are firmly committed to a two-track approach to Russia; strong defense, coupled with meaningful dialogue."
Russia has previously said it periodically sends Iskander missiles to Kaliningrad, but until now it has said these were routine drills.
Moscow has not linked the moves explicitly with what it says is a Nato military build-up on Russia's western borders.
Up to 300,000 Nato troops were put on alert earlier this month, amid rising tensions between Russia and the Baltic states.
In October, Nato was preparing to station 4,000 troops on the Russian border with the Baltic states in its biggest military build-up since the Cold War.
Some analysts predict an emboldened Moscow could become more assertive in eastern Europe after the election of Donald Trump as US president.
Mr Trump has said he wants closer ties with the Kremlin and has questioned the cost of protecting Nato allies.
Viktor Ozerov, chairman of the defence committee in the Federation Council, Russia's upper house of parliament, said Russia was forced to react to the planned US missile shield in eastern Europe.
"As response measures to such threats we will have... to deploy additional forces... This reinforcement includes deployment of S-400 and Iskander systems in Kaliningrad," the RIA news agency quoted Mr Ozerov as saying.
Additional reporting by ReutersReuse content