"This is one of a number of issues of concern," said the Pentagon official, who spoke on condition of anonymity.
A Pentagon official confirmed on Wednesday that Russia is believed to have moved short-range nuclear weapons, and said it was part of a "disturbing trend" that raised questions about Moscow's commitment to pledges it has made on arms control.
"This is one of a number of issues of concern," said the Pentagon official, who spoke on condition of anonymity.
Russia has flatly denied the report, first carried by The Washington Times newspaper.
The US Pentagon and State Department declined to confirm the report, saying it involved intelligence matters.
But State Department spokesman Richard Boucher admitted there were some concerns.
"It's something that we'll be talking about with the Russians, as we do on all arms control issues, but I don't think I can go beyond that."
Russian Defence Ministry sources denied the report, according to the Interfax news agency.
"This report can only be a political provocation," said Anatoly Lobsky, a spokesman for Russia's Baltic Fleet.
Weapons agreement
The Washington Times, citing unnamed US intelligence officials, said the movement of the new battlefield nuclear arms to Kaliningrad, a Baltic Sea port located in a Russian enclave between Poland and Lithuania, had been detected in June.
But the movement was not reported in an internal US Defence Intelligence Agency report until December, it said, citing the same unidentified officials.
The officials said the movement was "a sign Moscow is following through on threats to respond to Nato expansion with the forward deployment of nuclear weapons".
The newspaper said the weapons involved were probably for use on the new Tochka short-range missile, which is thought to have a range of about 70km (44 miles).
Russia and the US have a non-binding agreement to reduce arsenals of tactical nuclear weapons. In November Russian President Vladimir Putin proposed that both countries cut their nuclear arsenals to 1,500 warheads each.
Lithuania's Foreign Minister Antanas Valionis refused to comment on the report, saying it was not confirmed.
"Similar reports have been appearing several times a year but after raising public concern they after some time are usually forgotten."