Alexander Khramchikhin, an analyst at the Institute for Political and Military Analysis in Moscow, told Russia's state-run news agency RIA Novosti that Russia would like to know the rocket speed of the missiles planned for a European defense shield.
"If they (the United States) are really going to disclose data on (rocket) speed, that'll be the decisive argument," he was quoted as saying.
Khramchikhin said rocket speed would determine if the missiles posed a threat to Russia's arsenal.
Washington maintains its plans for a European defense shield are meant as a strategic deterrent against Iran. Moscow complains its own missiles would be vulnerable to the system.
U.S. authorities had said they were willing to share information about the missile defense shield with the Kremlin, though the Russian news agency said it wasn't clear what the White House was willing to share.
U.S. Secretary of State Hillary Clinton had stressed that a European missile defense system wouldn't upset the strategic balance with Russia.
Russian Deputy Foreign Minister Sergei Ryabkov had said Moscow might skip the upcoming NATO summit in Chicago unless it secures agreements over European missile defense.
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