WASHINGTON - The fact that the United States and Russia are unlikely to finalize a new nuclear arms control treaty before the current one expires tomorrow is "disappointing, but far from a tragedy," according to Stephen Young, a senior analyst with the Union of Concerned Scientists (UCS).

U.S. and Russian negotiators, meeting in Geneva, have been struggling to solve some remaining impediments to a new Strategic Arms Reduction Treaty (START). It would replace the original treaty, which was signed in July 1991, five months before the collapse of the Soviet Union.

"The deadline set by the two countries was extremely useful to motivate negotiators, but few thought a treaty could be negotiated and ratified by then," Young said. "The only question is what type of a bridging arrangement they will agree on and how long it will last."

The May 2010 Nonproliferation Treaty review conference, conversely, provides a deadline with actual consequences, Young added. If the START treaty is not signed and ratified by then, he said, the international community will be less likely to support the ambitious agenda that President Obama introduced in Prague last April and fleshed out this fall in a speech before the United Nations.

Negotiating a follow-on to START is important in part to preserve the treaty's strong set of verification measures allowing the two nations to monitor each other's nuclear arsenals. Without these measures, both countries would know much less about the other's arsenal. Both countries agree it is time to streamline some of the more expensive and time-consuming measures that were designed during the Cold War. But figuring out how to do that takes time.

The new treaty is expected to contain a limit of 1,500 to 1,675 deployed warheads for each country, a reduction that is roughly a third below current levels. The warhead limit in the original START treaty is 6,000; the new lower limit reflects both different ways of counting warheads under the two treaties and actual reductions in deployed warheads. How warheads are counted under the treaty has been a point of contention.

Russia is concerned that the United States has a larger "upload potential." Because no previous treaty has required the two countries to dismantle warheads, both countries store thousands of them that they withdrew from active service. Since the United States has more long-range missiles than Russia, and most can carry more warheads than they currently do, Russia is worried that the United States could reload warheads on its missiles and increase its deployed forces fairly quickly.

This perceived imbalance has led to a debate about which delivery vehicles should be counted under the treaty. Russia wants any limit on missiles and bombers to include those that do not currently carry nuclear weapons but could be converted back to nuclear use. This issue is not likely not to be resolved in detail in the current treaty, Young said.

This and other issues, such as the link between offensive and defensive weapons, as well as cutting non-strategic nuclear weapons, are expected to be taken up in future negotiations in coming years.

The United States, meanwhile, is concerned that Russia wants to field fewer missiles with more warheads, which would create incentives for Russia to launch first -- and quickly. This is because one missile with multiple warheads makes an attractive target, Young explained. In addition, to compensate for retiring old systems, Russia wants to deploy a new mobile missile, called the RS-24, which is a modification of an existing missile and therefore not allowed under START.

"All of these issues are significant," said Sean Meyer, UCS project manager for the UCS Nuclear Weapons Policy Initiative. "But both countries are realizing that these weapons are now a liability rather than an asset, and agreeing to reduce their arsenals further is the real focus."