With the clock ticking on a year-end deadline, President Barack Obama is pressing ahead with a top-to-bottom review of America's nuclear weapons to see how much the arsenal can shrink, as his negotiators are racing to wrap up a major new strategic arms control treaty with Russia. The review, in tandem with reinvigorated talks between Washington and Moscow, will help determine how much further the two nuclear superpowers will cut their arsenals after the 1991 Strategic Arms Reduction Treaty, or Start, expires Dec. 5. The last time the Pentagon reviewed its nuclear posture, in 2001, it concluded that the American military could get by with 1,700 to 2,200 nuclear warheads at the ready, a level the Bush administration found comfortable even as it demurred over a binding treaty with Russia. Now both sides want to go even lower. Russia is especially eager to lock in reductions, and Obama has made deep cuts a primary diplomatic goal. Their ambitions, and the impending deadline, make the Pentagon's review crucial, because it would help determine the bottom line, as well as which missiles, bombers and submarines to keep, how much to spend modernizing them and the implications of a changing world where small states, too, can acquire nuclear arms. But not everybody is at ease with the prospect of such rapid change. Several officials involved in the effort said powerful constituencies -- among arms specialists in the executive branch, Congress, the military and at the weapons laboratories -- had conflicting views of how to proceed. Although Obama has vowed that his long-term goal is eliminating nuclear weapons, there are significant disagreements about how fast and how deep reductions might be made while guaranteeing America's security in a world in which other nations maintain nuclear arsenals, others might be tempted to build them -- and bomb-making knowledge can never be erased. The shape of the arsenal also is a point of contention. Some military planners advocate building a new generation of safer and more reliable warheads, while some administration officials fear that reopening nuclear assembly lines would undermine their efforts at nonproliferation. The arms talks must deal not only with the limit on warheads, a ceiling that might be as low as 1,500 on each side, but also with arcane counting rules, verification measures and ancillary issues like the deployment of missile defenses. Tackling these extraordinarily complex issues at the same time on a tight schedule is an ambitious agenda, especially for an administration also trying to battle a deep economic crisis, overhaul the health care system and address global climate change. 'From a distance, it could look like, 'How do you do all that?' ' said Ellen O. Tauscher, the under secretary of state for arms control and international security. 'It's like the operation of a very high-end restaurant kitchen. It may look chaotic, but beautiful things come out of it.' Senior Defense Department officials said the nation's entire nuclear weapons architecture was under review, including such fundamental traditions as whether the nation still needs to maintain a triad of land-based missiles, submarine-launched missiles and bombers. Obama laid out his vision in April, declaring in Prague that he would 'reduce the role of nuclear weapons' and urge other countries to do the same, with the long-term goal of eliminating nuclear arms altogether. Under a framework agreement signed in Moscow this summer by Obama and his Russian counterpart, Dmitri A. Medvedev, the new treaty is to reduce the ceiling on long-range nuclear warheads to 1,500 to 1,675 within seven years, down from the current limit of 2,200 by 2012, under the separate Moscow Treaty signed in 2002. Total U.S. warheads reached more than 32,000 in the 1960s but dropped to 10,500 just before Start was signed in 1991. This year, the Federation of American Scientists reported that the United States had already reduced its deployed strategic nuclear warheads to 2,200, more than three years ahead of the Moscow Treaty schedule. Under the prospective new treaty, to be negotiated by December with follow-up talks to look at even deeper cuts, the total of all types of long-range delivery vehicles -- land-based missiles, submarine-based missiles and bombers -- would be limited to 500 to 1,100, down from the 1,600 now allowed. 'For some it is not enough of a cut, for others it is too much, too fast,' said one senior Defense Department official, who like other officials interviewed spoke on the condition of anonymity in order to describe the internal, classified discussion of the review. Another senior Pentagon official said the calculations not only were about specific numbers but also finding the right balance: 'So long as there are nuclear weapons in the world, how do we sustain a safe, secure and effective nuclear deterrent for us, and that can be extended to our allies? How do you define that?' As Washington hammers out an arms deal with Moscow, negotiators may also glean insights from the Russians that would help answer these questions, according to another senior State Department official. Senior officials involved in the review point out that the configuration of today's arsenal offers Obama some flexibility, even in advance of the final negotiations. Several hundred bombers and missile silos have been removed from nuclear use or decommissioned, yet still are counted under current treaty rules. The United States has just under 900 operational nuclear warhead platforms, meaning that Obama could easily give up significant numbers of missiles or planes in negotiations because they have already been taken out of nuclear service, officials said. The review will look closely at the contentious question of whether the arsenal should be used to threaten retaliation in case of catastrophic attack by an adversary using nonnuclear weapons, whether chemical, biological or even overwhelming conventional forces, against the United States or an ally. Reshaping the list of targets for America's nuclear warheads, officials said, also is under discussion. 'With the end of the Cold War and the development of new conventional technologies, the traditional purposes for U.S. nuclear weapons have become increasingly less relevant,' said Daryl G. Kimball, executive director of the Arms Control Association, an independent advocacy group. 'We can and should limit the role of our nuclear weapons to a core deterrence mission,' he added, noting that deterring attacks on the United States and its allies 'requires far fewer nuclear warheads and delivery systems.' Over the decades, however, the United States consciously maintained ambiguity in public statements about its nuclear policy -- when it would strike, what it would strike and in response to which actions by an adversary. 'We don't want to box our leaders in,' said a senior Pentagon official. 'They like to hedge against uncertainty. They like to have options.' 