Russia reserves the right to withdraw from the new strategic arms reduction treaty, if Washington's strategic missile defense shield has an excessive impact on the effectiveness of Russia's strategic nuclear forces, Russian Foreign Minister Sergei Lavrov warned Tuesday.

"Russia will have the right to pull out of the treaty on strategic arms reductions, if the quantitative and qualitative increase of the USstrategic missile defense potential starts to have a considerable influence on the effectiveness of the Russian strategic nuclear forces," Lavrov told reporters in Moscow, quoted by the Interfax news agency.

He added that Russia itself would determine the extent of such influence and that this was a specific precondition of Russia.

"Certainly, we will determine ourselves the scale of such influence. This will be specified especially in Russia's statement, which will make part of the package of the documents under a new strategic arms reduction treaty, and in a US reciprocal statement. This is our clear-cut position that will be taken into account," the Itar-Tass news agency cited the minister as saying.

Lavrov also urged Washington to abandon its plans to deploy elements of a missile defense shield in Bulgaria and Romania, saying that Washington's plan presently constitute no threat to Russian strategic interests, but Moscow did not rule out that the plans could eventually constitute a threat.

"If the strategic missile defense shield ... is estimated by our military specialists to be posing a risk for Russia's strategic nuclear forces, then we will have the right to use the conditions included in the (new arms reduction) deal," he said as quoted by the RIA Novosti news agency.

In February, Bulgaria and Romania said they were in talks with US President Barack Obama's administration on deploying elements of the US missile shield on their territories from 2015.

Lavrov also hoped the treaty would be submitted to US Congress and the Russian State Duma for ratification by the end of April.

Russian President Dmitry Medvedev and his US counterpart Barack Obama will sign the new document in the Czech capital of Prague on Thursday.

Under the new treaty, the warheads held by the two nuclear superpowers will be reduced to 1,500, about 30 percent lower than the previous treaty's limitation. Strategic offensive weapons will be based solely on the national territories of Russia and the United States.