Russia warned on Thursday that an expansion of the United States' planned missile shield could derail a new disarmament treaty intended to slash the two countries' nuclear arsenals.

Even as US President Barack Obama and his Russian counterpart Dmitry Medvedev signed the treaty in Prague, the Kremlin released a statement saying it would pull out if the US missile shield went too far.

"The treaty... can only function and be capable of life in conditions where there is no qualitative and quantitative expansion of a possible US missile defence system," the Kremlin said.

One of the "exceptional circumstances" that would undermine the treaty would be "an expansion of the possible US missile defence system that would threaten the potential of Russia's strategic nuclear forces," it said.

The Kremlin's threat soured an otherwise cordial day for US-Russian ties as the two presidents signed a successor to the 1991 Strategic Arms Reduction Treaty (START) after months of difficult negotiations.

Russian officials had previously said that Moscow would withdraw from the treaty if it felt Washington's anti-missile defences were expanding to the point where they threatened Russia.

Moscow and Washington have had a long-running and sometimes bitter dispute over US plans to counter incoming missiles, which bogged down the negotiations over the new START treaty.

The Kremlin's statement does not mean Russia will pull out of the treaty at any moment, said Russian Deputy Foreign Minister Sergei Ryabkov.

"We have a unilateral statement attached to this document where we say, in essence, that this treaty has been concluded under these kinds of circumstances," Ryabkov said.

"We are not suggesting that we will withdraw at whatever moment," he told reporters in Prague after the signing.

The United States says its shield is meant to protect against an eventual missile strike from Iran and poses no threat to Russia, but Moscow has remained sceptical about such assurances.

Moscow fears the shield could grow to the point where it could allow the United States to carry out a first nuclear strike against Russia with no risk of retaliation -- a concern rejected by Washington.

A White House official dismissed the importance of the Kremlin's statement and said the United States would proceed with missile defence.

"There is nothing particularly novel about this kind of unilateral statement," US National Security Council advisor Brian McKeon said in comments on the White House website.

"In fact, the United States issued its own unilateral statement, indicating that it plans to continue to develop and deploy its missile defence systems in order to defend itself," he added.

Russia's threat to quit the treaty is hollow because it needs the agreement to maintain nuclear parity with the United States as its Soviet-era arsenal ages, said Russian defence analyst Pavel Felgenhauer.

"Russia does not plan to leave the treaty.... The treaty is more important for us than it is for the Americans," said Felgenhauer, a defence commentator for the Russian opposition newspaper Novaya Gazeta.

"US missile defence will only become a threat to Russia's nuclear deterrent maybe 20 years from now, or maybe never," he said.

"This type of statement is for internal consumption, to show that we have held our position. It is for patriots within Russia only."