Talks between the U.S. and Russia over a new anti-ballistic missile system for Europe are stuck on a key point, with Moscow demanding to jointly run the system and the U.S. refusing to yield.

Russia is insisting on shared control of the missile defense program with the U.S. and NATO, which President Barack Obama has flatly opposed because it would essentially give Russia responsibility for protecting NATO from nuclear missile threats. The U.S. is offering Moscow a more limited role.

After years of opposition, Russia agreed last fall at least to talk about cooperating on the anti-ballistic missile plan for Europe, which the U.S. says may one day be needed if Iran develops nuclear weapons. Experts from both sides are scheduled to report on details of the proposal to defense ministers in July.

But Moscow has refused so far to budge from its demand for joint control, and has been keeping up the rhetorical pressure. In late November, Russian President Dmitry Medvedev said if the U.S. and NATO can't reach an agreement on missile defense Russia may deploy new offensive weapons, triggering a new arms race.

Early this month, a Russian deputy foreign minister warned anything less than a "joint system" could lead Russia to withdraw from the recently-ratified New START treaty and to "take other measures, including military-technical measures."

Russia's U.S. Ambassador, Sergey Kislyak, told an industry-sponsored conference here this week that the two sides still haven't come up with a good understanding of how a joint program would work, warning that his country wasn't interested in "cloning" decisions already made by the U.S.

Referring to Russian fears that the missile defense system could target Russian warheads, Kislyak said Moscow is determined to maintain a strategic nuclear balance with the West.

"We want to be reassured that whatever you do there doesn't undermine the stability of deterrence, because deterrence is still with us," Kislyak said Wednesday at the Nuclear Weapons Monitor Nuclear Deterrence Summit in Washington. "We haven't reached a state ... between our two countries that would allow us to abolish it. We would like to see it happen. But that's going to be a long way (off)."

The U.S. and NATO have proposed sharing radar and other early warning data, but Assistant U.S. Secretary of State Rose Gottemoeller, the top U.S. arms control negotiator, told the industry summit that Obama has decided that "NATO will protect NATO, and that's the bottom line as far as we're concerned."

The issue could make or break the deal.

"The hardest question on missile defense in the end is who pulls the trigger," said Steven Pifer of the Brookings Institution, a veteran of U.S. arms control negotiations and former U.S. ambassador to Ukraine. He nevertheless thinks that an agreement can be reached.

The White House had hoped that the New START treaty limiting U.S. and Russian strategic weapons, which took effect Feb. 5, would be a springboard to further arms deals, including deeper cuts in strategic forces as well as reductions in short-range nuclear weapons and non-deployed warheads.

U.S. officials say that new limits on the strategic arsenals of the U.S. and Russia, which between them control 90 percent of the world's deployed nuclear arms, are crucial to efforts to halt the spread of those weapons and promote disarmament worldwide.

Both U.S. and Russian officials have been vague about the details of Russia's proposal for a joint missile defense system, which Medvedev has called a "sectoral" defense.

But independent Russian military expert Alexander Golts said the plan would give Russia responsibility for intercepting missiles headed across its territory toward Europe, while NATO would be responsible for missiles headed across its territory toward Russia.

The apparent aim is to ensure that NATO's interceptors aren't aimed at Russian ICBMs, where they might cripple Russia's ability to respond to a first strike from the West.

Golts said the Kremlin may recognize that the U.S. could never agree to such a scheme but has adopted this position as a bargaining tactic. Russia's bottom line, he said, may be an agreement by the U.S. not to deploy ship-based missile interceptors in the Arctic region, where in theory at least they could shoot down Russian strategic ICBMs headed for U.S. targets.

"If Americans are honest when they say that this missile defense is not aimed at Russian nuclear potential, why not?" Golts said.

Experts say U.S. missile defense systems aren't capable of posing a serious threat to Russia's nuclear missiles, and are intended to protect against future any missile threats from Iran and North Korea. But Russia has said that it is concerned the current systems will develop into a full-scale missile shield.

Russia isn't the only country skeptical of U.S. missile defense goals.

"The Chinese are very, very concerned about missile defenses," said Sharon Squassoni of the Center for Strategic and International Studies, a Washington think tank. "A moderately effective missile defense, which we don't have yet, could really neutralize their nuclear capability and they know that. .We have to do this with the Russians because we're going to have to do this with the Chinese later on."

Despite the impasse, several current and former U.S. officials expressed confidence the U.S. and Russia eventually will find a compromise and strike a deal.

"We believe that we both have systems that can augment each other," Undersecretary of State for Arms Control Ellen Tauscher said this week. She added that the New START treaty had created an environment "where we can work to make sure that NATO is protecting NATO, Russia is protecting Russia."