European countries bordering Russia's territory of Kaliningrad say they are worried at reports that Moscow has put nuclear-capable missiles there.
Lithuania and Poland both issued statements of concern.
Russia has not confirmed the report but insists it has every right to station missiles in its western-most region, the BBC reports.
Moscow has long threatened to move Iskander short-range missile systems to Kaliningrad in response to the United States' own European missile shield. Russia sees the missile shield as a threat to its nuclear deterrent.
It was one of the biggest sources of confrontation between Moscow and Washington during the presidencies of George W Bush and Vladimir Putin.
US President Barack Obama tried to "reset" relations with Russia, and the shield system was revised - but it survived in a different form and continued to antagonise Russia.
The US insists that the missile shield is not aimed at Russia but designed to defend Europe from attack from "rogue states" - assumed to include Iran.
Lithuania's Defence Minister Juozas Olekas said: "I am worried about signals that Russia is about to modernise missile systems it has deployed in Kaliningrad."