A U.S.-led NATO missile defense plan that Washington says is aimed at deflecting potential Iranian threats will break existing nuclear parity with Russia and prompt it to retaliate, President Dmitry Medvedev warned Friday.

Moscow rejects Washington's claim the plan is solely to deal with any Iranian threat and has voiced fears it will eventually become powerful enough to undermine Russia's nuclear deterrent.

"No one has explained to me why we should believe that the new missile defense system in Europe isn't directed against us," Medvedev said in a speech at a security conference, adding that the shield will "break the nuclear parity."

NATO has said it wants to cooperate with Russia on the missile shield, but has rejected Moscow's proposal to run it jointly. Without a NATO-Russia cooperation deal, the Kremlin has sought guarantees from the U.S. that any future missile defense is not aimed at Russia and threatened to retaliate if no such deal is negotiated.

"I will say honestly that no matter how warm relations between me and my colleagues are, no matter how advanced relations between Russia and NATO member states are, we will have to take that into account and, under certain circumstances, respond," Medvedev said.

Earlier this week, he told the top Russian military brass that the armed forces must prepare to counter U.S. missile defense plans even as talks between Moscow and Washington are continuing.

"By 2017-2018, we must be fully prepared, fully armed," Medvedev said, referring to his earlier threat to aim missiles at the U.S.-led NATO missile shield if no agreement is reached.

Speaking at Friday's conference, he reaffirmed that Russia isn't "shutting the door to dialogue" but warned that "time is running out."

"It's in our mutual interests to quickly reach mutually acceptable agreements," he said.

Dmitry Rogozin, a deputy prime minister in charge of the military industries, was more hawkish in his remarks at the conference, saying the NATO shield has an "openly anti-Russian vector."

"The system that is being developed is intended to intercept heavy intercontinental missiles blasting off from the Russian territory," Rogozin said.

"Missile defense isn't the best way to ensure security," he added. "Those who are smart know that the defensive arms race is no better than the offensive arms race. Strengthening of the shield entails strengthening of the sword."

Rogozin claimed the new Russian missiles have been fitted with systems that would allow them to penetrate any prospective missile defense. "They would allow Russia to feel absolutely calm, even if the missile defense becomes global and affects our interests," he said.

Tensions over the missile shield, which long have tarnished ties between Moscow and Washington, are expected to flare again in May when Vladimir Putin heads to the U.S. for the Group of Eight summit shortly after being sworn in for his third presidential term.

NATO Secretary-General Anders Fogh Rasmussen said this week that Putin would be unlikely to attend NATO's summit in Chicago that follows the G-8 summit. The NATO chief cited Putin's busy domestic political calendar, but many observers believe the Russian leader would not come to Chicago because of the missile defense rift.

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Sofia Javed and Mansur Mirovalev contributed to this report.