JERUSALEM — Fresh evidence has emerged that suggests Iran has renewed work on developing nuclear weapons, according to Western security sources.

Experts responsible for monitoring Iran’s nuclear program have discovered that enough enriched uranium, which if processed to weapons grade level could be used to make up to six atomic bombs, has disappeared from the main production facility at Isfahan.

American spy satellites have identified a number of suspicious sites, which the Iranians have not declared to nuclear inspectors, that intelligence officials believe are being used for covert research.

The discoveries emerged as it was learned that Israel had asked America for military supplies, including “bunker buster” bombs and refueling planes, suitable for attacking Iranian nuclear installations.

The Israeli paper, Haaretz, reported yesterday that Israel had also asked for permission to use an air corridor through Iraqi airspace, currently controlled by America, to Iran.

So far the requests have been turned down by Washington, which is not as keen as Israel to consider a military strike against Iran.

But concern that Iran has resumed work on building atomic bombs has deepened following the disclosure that large quantities of uranium has gone missing.

The Isfahan complex, which enriches raw uranium “yellow cake” into material that can be used for either nuclear power or atomic weapons, is supposed to be subject to close supervision by the Vienna-based International Atomic Energy Agency.

But the Iranians only allow IAEA inspectors access to the final stage of the production process, where the uranium in gas form — UF6 — is stored.

By conducting a careful study of the amount of material stored at Isfahan, and the amount of “yellow cake” known to have been processed at the plant, nuclear experts believe between 50 to 60 tons of uranium — which if enriched to weapons grade level would be sufficient to produce five or six bombs — has disappeared.

IAEA officials believe the Iranians have removed the uranium at a stage in the production process that is not under their supervision.

A nuclear official said, “If Iran’s nuclear intentions are peaceful, then why are they doing this?”

IAEA officials have no idea where the missing uranium is being stored, but suspect it could be held at one of several installations spotted by American spy satellites.

The Iranians will be asked to account for the missing enriched uranium when the IAEA’s board of governors meets later this month.