WASHINGTON -- To generations who came of age during the Cold War, the nuclear arms agreement announced Monday by U.S. and Russian officials has the ring of a once-impossible dream.

Thousands of nuclear weapons would be removed from arsenals that defined decades of hostility between the United States and the former Soviet Union.

But the agreement encountered immediate criticism that it fails to address the more realistic, post-Cold War threats that the United States faces. In particular, arms control experts said the treaty would raise the likelihood that warheads would find their way into the hands of terrorists or rogue nations because the Bush administration insisted that both countries be allowed to achieve reductions by storing--rather than dismantling--their nuclear weapons.

As a result, thousands of Russian warheads that might have been permanently disabled under a more aggressive treaty could instead be moved into storage facilities whose security is rated uncertain at best, even by the administration's own recent intelligence assessments.

"Reducing nuclear weapons on both sides is a good thing," said Ivo H. Daalder, a senior fellow at the Brookings Institution, a nonpartisan think tank in Washington. "But we may be no more secure after this agreement has been implemented than we are today."

Under terms of the treaty, which must be approved by the U.S. Senate and the Russian Duma, the two countries would over the next decade reduce their stockpiles by two-thirds, to 1,700 to 2,200 warheads apiece.

The agreement leaves it to the countries to determine how they would make those reductions. A White House official who asked not to be identified said the Pentagon is responsible for working out details of U.S. compliance. Some of the U.S. weapons would be dismantled, the official said, but others would be placed in "deep storage" or set aside as "operational spares."

Weapons placed in the latter category would not be available for immediate use on submarines or bombers or in silos, but they could be quickly redeployed.

The White House has not spelled out specific scenarios in which it envisions a redeployment of stored nuclear weapons, and arms control advocates have said it's hard to envision circumstances in which 2,200 warheads would be inadequate.

But the Bush administration insisted that the treaty allow flexibility for "uncertain security environments" in the future. The three-page accord also contains a clause allowing either side to exit the agreement entirely with three months' notice.

That aggressive posture scored points with conservatives, making the proposed treaty palatable to some who have questioned the need for any arms control negotiations in an era when the United States is the world's sole superpower.

"This is the least bad outcome," said Frank Gaffney Jr., president of the conservative Center for Security Policy and a former Pentagon official.

The United States and Russia had been pursuing the treaty for much of the last year and until recently disagreed sharply over how far to go in taking weapons offline.

Russia, whose economic troubles have strained its ability to maintain its arsenal, had pressed for language that would have required weapons to be dismantled. But Russia had little bargaining power in negotiations that often highlighted the growing disparity between the two countries' fortunes since the end of the Cold War.

Though Russia preferred dismantling weapons, President Vladimir V. Putin is now likely to feel politically compelled to maintain parity with the United States by storing an equal number of warheads, experts said.

"If the United States is storing rather than dismantling its weapons, it will be hard for the Russians to [do otherwise]," said Robert Einhorn, a former U.S. assistant secretary of State for nonproliferation.

But Russia's ability to safeguard its nuclear arsenal has come under increasing doubt.

A report by the National Intelligence Council--a panel representing U.S. spy agencies--concluded that Russia's security system "was designed in the Soviet era to protect weapons primarily against a threat from outside the country and may not be sufficient to meet today's challenge of a knowledgeable insider collaborating with a criminal or terrorist group."

The report says that security at Russian facilities is already being tested. Russian authorities, according to the document, have "twice thwarted terrorist efforts to reconnoiter nuclear weapons storage sites" in recent years.

Many of Russia's nuclear weapons storage sites "remain off-limits to U.S. officials," meaning their security conditions have never been evaluated, the report said.

And economic turmoil in Russia continues to erode its ability to safeguard its arsenal. In some cases, guards at nuclear facilities have gone months without pay, and others have launched hunger strikes to demand better working conditions.

All of which makes Russia's vast nuclear weapons infrastructure vulnerable to organized crime and terrorist groups who could theoretically sneak out a warhead in a small pickup truck, experts said.

"The longer those warheads sit around in Russia, the greater the chance that they get lost or sold," said Tom Z. Collina, director of the global security program at the Union of Concerned Scientists.

Dismantled warheads also pose a security risk. Their component parts could still be used to reconstruct a warhead or a "dirty" bomb in which radioactive material is dispersed by a traditional explosive device. But the threat would be greater if thousands of Russian warheads went into storage intact, Collina and others said.

"Certainly it's good to take the arsenals down by two-thirds," Collina said. "But this treaty leaves unanswered the question of what happens to the warheads once they're off the missiles."