Russia's argument against a U.S. national missile defense system boils down to asking the United States to remain vulnerable even as Russia provides weapons technology to U.S. adversaries such as Iran, Defense Secretary Donald H. Rumsfeld said Thursday.

Rumsfeld, freshly returned from talks in Moscow, said the Bush administration still plans to withdraw from the 1972 Anti-Ballistic Missile treaty should the Russians not agree to allowing the United States to develop a nationwide defense against incoming missiles.

"A treaty ... ought not to stand in the way of protecting the population centers of the United States," he said in an interview on PBS's "NewsHour with Jim Lehrer." "A policy of vulnerability in the 21st century ... would be a terrible mistake."

The Russians, and many U.S. allies, say breaking the ABM treaty could trigger an arms race by nations who fear their current weapons would be neutralized by a missile defense system.

But Rumsfeld called it a treaty "structured on hostility."

Rumsfeld also noted that unlike U.S. cities, Moscow already has a missile defense shield. Moscow's defense is permissible under the ABM treaty, while the national defense proposed by the Bush administration is not.

Under the treaty, countries may build a local defense against ballistic missiles in a single location. During the Cold War, the Soviet Union built defenses to protect Moscow. It would function by detonating nuclear weapons in the path of incoming U.S. missiles. Those defenses are still operating, unlike a similar U.S. system that was built to protect the missile silos.

Rumsfeld also criticized Russia's sale of weapons technology to U.S. adversaries such as Iran, one of the countries the United States says its missile defense system would be built to stop.

"There's no question they are working with Iran on their nuclear capability," he said.

Rumsfeld's visit was among the first steps in implementing a recent agreement between President Bush and Russian President Vladimir Putin to have simultaneous talks on expanding defensive systems and reducing missiles and nuclear warheads.

The defense secretary said formal negotiations with the Russians regarding changing or ending the ABM treaty -- negotiations the Russians want -- are unnecessary. "You negotiate with an enemy," said Rumsfeld, who said all that is needed are less formal consultations.

Rumsfeld was also unconcerned by the suggestion of some Russian officials that they may begin deploying multiple warheads on its intercontinental ballistic missiles, should the U.S. start building missile defenses.

As long as neither side increases its total number of nuclear warheads, he said, it's not important whether those warheads are delivered one by one or as part of a multiple-warhead delivery system.

"It sounds bad," he said. "But what really counts ... is the total number of weapons."