WASHINGTON - The Bush administration has yet to make the case for building a new generation of replacement warheads and "the role of nuclear weapons" in a post-Cold War, post-9/11 world, a panel of nuclear-weapons experts said yesterday.

Development of the new warhead, the first in two decades, could have "international impacts" if critics view it as a new weapon rather than a replacement for the current aging stockpile, said the scientists, including three former directors of the government's nuclear-weapons research laboratories.

Some countries could see the warhead "as contrary to both the spirit and letter" of the Nuclear Nonproliferation Treaty "unless explicit and credible efforts to counter such assertions are made," said the panel, which was convened by the American Association for the Advancement of Science to study the warhead plan.

The scientists also said in a report that it was impossible to estimate the cost of the warhead-modernization plan, or to ensure that Energy Department claims of cost savings would ever be achieved. Proponents of the program may be "overselling" the eventual benefits, the report said.

Thomas D'Agostino, head of the department's National Nuclear Security Administration, which is spearheading the warhead project, called the report "a valuable contribution" to the discussion. He said the recommendations were consistent with the agency's plans to move forward.

The administration argues that the new warhead is needed because of concerns about maintenance and future reliability of the existing warheads in an era of no underground nuclear testing. It would be designed to be more robust and more easily maintained and to include improved safeguards to prevent potential use by terrorists, its proponents maintain.

Reaction in Congress to the administration's proposed Reliable Replacement Warhead, or RRW, has ranged from skepticism to sharp opposition in recent weeks.