VIENNA: The United States accused Iran on Friday of a "provocative expansion" of sensitive nuclear activity and voiced concern that some nuclear material may have been diverted to suspected weapons-related development research.

U.S. Ambassador Glyn Davies made the allegation in a statement to the 35-nation board of the U.N. nuclear watchdog, which later on Friday was expected to pass a resolution rebuking Iran over mounting concerns that it is seeking the capability to produce atom bombs.

"Iran's covert attempts to develop undeclared pathways for the production of nuclear material and work on nuclear weapons technology leaves little doubt that Iran, at the very least, wants to position itself for a nuclear weapons capability," Davies said, according to a copy of his speech.

Last week, an International Atomic Energy Agency report presented a stash of intelligence indicating that Iran has undertaken research and experiments geared to developing a nuclear weapons capability.

Iran says it is enriching uranium only as fuel for nuclear power plants, not atomic weapons. It has dismissed the details in the IAEA report obtained mainly from Western spy agencies as fabricated, and accusing the IAEA of a pro-Western slant.

Davies denounced Iranian plans to move higher-grade uranium enrichment work to an underground bunker near the city of Qom.

"Stockpiling uranium enriched to near 20 percent is a dangerous provocation because it positions Iran to move closer to the production of highly enriched uranium in a shorter period of time," Davies said.

Enriched uranium can be used to power plants, which is Iran's stated aim, or provide material for weapons if refined further to a level of around 90 percent, as Western states suspect is Iran's ultimate aim.

Davies also singled out a finding in last week's IAEA report of a "discrepancy" of nearly 20 kg (44 lb) of nuclear material at a site in Tehran, the Jabr Ibn Hayan Multipurpose Research Laboratory.

The report said U.N. inspectors carrying out an inventory check of natural uranium metal and process waste at the facility measured 19.8 kg less than the operator's declaration.

The IAEA said it was working with Iran to resolve the issue and Iran's envoy to the IAEA, Ali Asghar Soltanieh, last week dismissed it as "absolutely not an issue."

Davies said the issue required "immediate" resolution, citing information indicating that "kilogram quantities" of natural uranium metal had been available to Iran's military programme.

"It remains to be seen whether this discrepancy could ultimately represent another piece in the puzzle the IAEA is assembling to show Iran's nuclear weapons-related activities," Davies said.

Experts say such a small quantity of natural uranium could not be used for a bomb, but that the metal could be relevant to weapons-relevant tests and research. (Reporting by Fredrik Dahl and Sylvia Westall; Editing by Mark Heinrich)