Russia on Tuesday blasted US security moves in Europe, saying even toned-down missile defence plans were intended to weaken Russia and that the US-led NATO alliance remained a "serious" threat.

"The development and deployment of missile defences is aimed against the Russia Federation," General Nikolai Makarov, chief of staff of the Russian armed forces, was quoted as saying by domestic news agencies.

"The development of these missile defence systems without question weakens our potential nuclear deterrent."

Makarov, Russia's top military officer regarded as a reformer and known for straight talking, derided as disingenuous claims by Washington and its allies that plans to site missile defence elements in Europe were unrelated to Russia.

"This is not the case," he said.

The US State Department, however, attempted to dampen Moscow's concerns, insisting that Russia was not the target of the new scheme.

"The emerging missile defence architecture in Europe is not aimed at Russia, but rather the emerging threat from Iran," said State Department spokesman Philip Crowley.

"We continue to discuss ways in which we can cooperate with Russia on missile defence."

Separately, Nikolai Patrushev, the influential chief of Russia's national security council and former director of the FSB intelligence service, said NATO remains the top foreign military threat for Russia.

"We deeply doubt that we will be safer as a result of NATO enlargement," Patrushev was quoted by Russian news agencies as saying.

"For us, the alliance represents a threat and a fairly serious one."

His comments came after NATO Secretary General Anders Fogh Rasmussen voiced surprise Saturday that Russia, in a key recent strategy document, had named the Atlantic alliance as its "chief external military threat."

The sudden burst of vitriol from Moscow came as Russia and the United States pursue efforts to thrash out a new nuclear disarmament treaty to replace the landmark 1991 START accord that expired in December last year.

Moscow has reportedly insisted that any new pact regulates both strategic offensive missiles and the anti-missile systems designed to thwart them, a linkage that the United States has never favored.

Makarav said the START talks, which have dragged long past deadlines, were stuck on that issue.

In September, US President Barack Obama shelved plans -- fiercely opposed by Moscow -- to site elements of a high-tech US missile shield in Poland and the Czech Republic, a plan pursued vigorously by his predecessor, George W Bush.

Moscow initially welcomed that decision amid talk of a wide-ranging reset in strained US-Russian relations but reserved judgement on Obama's simultaneous announcement of plans to pursue a more modest missile defence scheme.

Last Thursday, Romanian President Traian Basescu said his country -- like Poland and the Czech Republic, a former east bloc satellite of Moscow -- had agreed to host ballistic missile interceptors as part of a new US shield.

Basescu said the shield was not aimed at Russia, prompting Russia's foreign ministry to denounce what it called "a serious matter which we will be analysing with care."

Meanwhile, the White House on Tuesday denied that differences with Russia on the US anti-missile shield was holding up an agreement on a new strategic nuclear weapons reduction treaty with Moscow.

"The notion that this is in some way an impediment to what is going on with START (Strategic Arms Reduction Treaty) is not true," White House spokesman Robert Gibbs told reporters.

Russia has long voiced worry over NATO's englargement to take in eastern bloc countries that were until a generation ago part of the Warsaw Pact led by Moscow.

Obama's predecessor in the White House pushed strongly for the Atlantic alliance to extend membership to Ukraine and Georgia, both once part of the Soviet Union itself, but the alliance has put those ideas on ice.

Relations between NATO and Russia were frozen over the August 2008 war in Georgia war and have only just begun to thaw, amid efforts to focus on common concerns like the conflict in Afghanistan and the fight against "terrorism".