Washington and NATO say the missile shield would protect Western nations from missile attacks from possible nuclear powers such as North Korea and Iran. Moscow says the shield could weaken its own nuclear capabilities and proposes collaborating on a joint system.
U.S. Ambassador to Russia Michael McFaul told Russia's state-run news service RIA Novosti that U.S. national security interests were a primary concern in missile defense plans.
"We are going to accept no limitations on that whatsoever because the security of our people, of our allies, is the No. 1 top priority," he said.
Working with Moscow, said McFaul, was at the top of the agenda for U.S. President Barack Obama should he win a second term in office in November elections.
"The president believes that this is an issue where we can turn from confrontation to cooperation because we have no interest in building a missile defense system against Russia's nuclear arsenal," he said.
Obama was overheard telling Russian President Dmitry Medvedev during a nuclear security summit in Seoul last month that he'd have "more flexibility" to tackle missile defense issues the election.
The Obama administration defended that statements by saying it would be difficult to have a candid conversation about a complex issue like missile defense during a presidential campaign season.