MOSCOW (AP) - Russia will replace single nuclear warheads on some of its strategic missiles with multiple warheads, Russian news agencies reported Friday, allowing Moscow to modernize its nuclear arsenal while building fewer new missiles - and spending less.

In theory, the shift would also make it easier for Russian nuclear weapons to evade a U.S. missile defense system.

"In the near future we will begin to substitute the single warheads on Topol-M intercontinental missiles with multiple warheads," the Interfax-Military News Agency quoted Gen. Nikolai Solovtsov, commander of Russia's Strategic Rocket Forces, as saying Friday.

"This makes the task of replacing aging missiles much easier," said Alexander Pikayev, a Moscow-based defense analyst who is co-chair of the Committee of Scientists for Global Security.

On Thursday, President Vladimir Putin said the deployment of Topol-M missiles on mobile launchers was a "serious step forward in strengthening Russia's defense capability."

Capable of hitting targets more than 6,000 miles away, the Topol-M missiles have so far been deployed only in silos. The mobile version of the missile, mounted on an off-road vehicle, is harder to locate and destroy.

The United States has not deployed similar mobile launch systems, but it has better access to oceans and can concentrate its nuclear missiles in submarines, Pikayev said.

Johns Isaacs, executive director of the Council for a Livable World in Washington, said Friday that the deployment would not change the strategic balance between Russian and U.S. nuclear arsenals.

"It's a reflection that the Russians as well as the Americans continue to update their forces with weapons they won't use and don't need," Isaacs said. "Adding a few more here or there is not going to make any difference in the balance of power, the state of the world, peace on earth, or good will toward men."

During the economic shocks of the 1990s, Russia was slow to modernize its nuclear weapons systems.

The military has commissioned just over 40 of the Topol-M missiles since 1997, and aging Soviet-era missiles form the backbone of the nation's nuclear capability.

In 2002, Putin and President Bush signed a treaty obliging both sides to cut the number of strategic nuclear weapons by about two-thirds by 2012, down to between 1,700 and 2,200 missiles each.

When the treaty was signed, many analysts said the number of Russian nuclear weapons could fall far below the number set by the treaty.

However, the recent oil boom allowed the Kremlin to increase military spending and speed modernization.