WASHINGTON - The National Institutes of Health official overseeing the implementation of President Bush's policy on embryonic stem cells suggested yesterday that the program was delaying cures, an unusually blunt assessment for an executive-branch official.

Story Landis, director of NIH's National Institute of Neurological Disorders and Stroke and interim chair of the agency's stem-cell task force, spoke plainly under questioning from senators. When Sen. Edward M. Kennedy (D., Mass.) asked her how the policy was affecting medical research, she said: "We are missing out on possible breakthroughs."

The ability to work on newly derived stem-cell colonies - precluded from federal funding under the Bush plan - "would be incredibly important," she added.

Landis also declared that "science works best when scientists can pursue all avenues of research. If the cure for Parkinson's disease or juvenile diabetes lay behind one of four doors, wouldn't you want the option to open all four doors at once instead of one door?"

The House this month passed a bill that would loosen Bush's restrictions, and the Senate is poised to do so in February. Bush has promised to veto the bill.