LOS ANGELES — The Energy Department is holding 324 metric tons of bomb-grade uranium at the same time that the Obama administration is urging foreign nations to reduce or eliminate their stores of the material, according to a report to be released Tuesday by the nuclear watchdog group Project on Government Oversight.

The Washington-based group wants the administration to declare a part of the U.S. inventory of highly enriched uranium as surplus and increase the amount that is blended down each year into commercial reactor fuel.

The inventory began to swell years ago after the U.S. agreed to a series of nuclear arms accords resulting in the decommissioning of thousands of nuclear warheads. The U.S. stopped making highly enriched uranium after the end of the Cold War.

The Energy Department’s National Nuclear Security Administration defended the rate at which it is blending the uranium into commercial fuel, noting the difficulty and cost of the process. It did not comment on the size of the surplus, which is classified.

The NNSA also said that it is not sending out a contradictory message by maintaining the surplus. It said that its facilities are secure from terrorists and that the agency provides technical assistance to other nations when they give up their bomb materials.

But the issue is drawing fresh scrutiny from outside nuclear nonproliferation groups and Congress.

“”The U.S. would be on higher moral ground if we clearly articulated that we are working to minimize our use of highly enriched uranium,”” said Joan Rohlfing, president of the Nuclear Threat Initiative, a nonpartisan group. “”It should be the norm that every country with these materials publishes their status.””

Staff from the House Armed Services Committee is preparing to go to a new $500 million Tennessee facility where the uranium is stored. A spokesman for the Republican staff said they want to ask why existing HEU surpluses are being “”downblended”” at a slower than expected rate.

Frank N. von Hippel, a Princeton University nuclear weapons expert and co-chairman of the International Panel on Fissile Materials, said, “”We are awash in surplus HEU.”” But von Hippel makes a more conservative estimate of the surplus, putting it at about 60 metric tons.

The Navy uses highly enriched uranium to power its submarines and aircraft carriers. Under an earlier declaration, 160 metric tons of the material was set aside for future Navy needs, enough for 25 to 50 years of operation.

Small amounts of the material are also used by research reactors to produce medical isotopes and by the NASA to power deep space probes.