Dmitry Medvedev, the Russian president, has threatened to deploy missiles to target the US missile shield in Europe if the US fails to assuage Russian concerns about its plans.
The warning reflects deep cracks in US-Russian ties despite President Barack Obama's efforts to "reset" relations with the Kremlin.
Medvedev said he still hopes for a deal with the US on missile defence, but he strongly accused the US and its NATO allies of ignoring Russia's worries. He said Russia will have to take military countermeasures if the US continues to build the shield without legal guarantees that it will not be aimed against Russia.
The US has repeatedly assured Russia that its proposed missile defence system would not be directed against Russia's nuclear forces.
The US missile-defence dispute has long plagued ties between Moscow and Washington. The Obama administration has repeatedly said the shield is needed to thwart off a potential threat from Iran, but Russia fears that it could erode the deterrent potential of its nuclear forces.
Al Jazeera's Robin Forestier-Walker reports from Moscow.
Source: Al Jazeera and agencies