The United States, Poland and three Baltic countries have expressed alarm at Russia's deployment of nuclear-capable missiles to the exclave of Kaliningrad, which borders the European Union.
The Kremlin confirmed that it had placed Iskander-M missiles in its westernmost region after reports that spy satellite photographs showed the weapons stationed near the Polish border.
The missiles, which can be armed with nuclear warheads, have a striking range of hundreds of kilometres and have been deployed by Russia in response to Nato's development of a missile defence shield against Iran.
The confirmation of the siting of the missiles also comes as tensions rise between the EU and Russia after the suspension of a European trade agreement with Ukraine following pressure from Moscow.
Marie Harf, a spokesman for the US State Department, said last night: "We've urged Russia to take no steps to destabilise the region."