The International Atomic Energy Agency said Monday that Iran is still failing to cooperate in their probe of Tehran's alleged nuclear arms program.

Iran has effectively stonewalled U.N. nuclear inspectors attempting to resolve longstanding concerns about "possible military dimensions" to Tehran's nuclear program, a new report by the nuclear watchdog's top inspector says.

IAEA Director General Mohamed ElBaradei reiterated his agency's previous complaints about Iran's cooperativeness in his latest report to the IAEA Board of Governors, obtained Monday by FOX News.

In unusually strong language, ElBaradei pointed to "the doubts which naturally arise, in light of all the outstanding issues, about the exclusively peaceful nature of Iran's nculear program."

ElBaradei cited Iran's refusal since April to provide more information about a broad range of military activities which could either be geared toward the production of a nuclear warhead or which may simply reflect, as Tehran claims, the work of a normal conventional weapons program.

The report also said that — through uranium enrichment — Tehran now has amassed a third of the amount of enriched uranium it could reprocess into the material serving as the fissile core of a nuclear weapon, should it choose to do so.

The U.S. and its allies say Iran wants to develop its enrichment program for its weapons applications. But Tehran insists it seeks the technology only to create nuclear fuel, and IAEA oversight and inspections of the Islamic Republic's known enrichment program has not come up with any indications that contradict what Tehran says.

The six-page report confirmed that Iran continues to expand its uranium enrichment program in defiance of three sets of U.N. Security Council sanctions imposed in attempts to force Tehran to mothball such activities.

The document said Iran was now either fully or partially operating 6,000 centrifuges at its cavernous underground facility at Natanz. Beyond those machines, which spin uranium gas into enriched uranium, it was testing 12 more advanced prototypes at its aboveground experimental site at Natanz, a city about 300 miles south of Tehran.

To date, Iran had enriched about 1,000 pounds of low enriched uranium suitable for nuclear fuel, the report said. Asked to put that figure into context, U.N. officials said Tehran would need three times that amount to begin the process of enriching to the level needed to produce a nuclear weapon.

Running smoothly, 3,000 centrifuges could produce enough nuclear material for a bomb within 18 months, were they configured for that function.

Iran's refusal to end enrichment has been the main trigger for the Security Council sanctions and continues to be the overriding concern for Washington and others accusing Tehran of wanting to make a bomb.

But with Tehran repeatedly saying it was not planning to stop its enrichment program — and even giving progress reports on its expansion — the conclusions on enrichment revealed little new. That left the IAEA's acknowledgment — that it had been stonewalled in months of efforts to shed more light on the allegations of past secret Iranian experiments geared toward developing nuclear arms — the most striking part of the report.

Since its last report in May, "the agency ... has not been able to make any substantive progress," the document said. It called the impasse a matter of "serious concern."

If Iran continues to block investigators, the IAEA "will not be able to provide credible assurance about the absence of undeclared nuclear material and activities in Iran," the report said.

Anticipating U.S. condemnation, Iranian Foreign Ministry spokesman Hasan Qashqavi said in Tehran before the report's release that his country expected the IAEA not to bow to U.S. "pressures."

Iran confirmed the IAEA report, which details a lack of progress on the nuclear front, but vowed to cooperate with the U.N. watchdog's investigation, Reuters reported.

The White House scolded Iran's stonewalling of investigators, and called on Tehran to stop enriching uranium and other reprocessing activities or face the possibility of further sanctions.

"This report shows once again that Iran is refusing to cooperate with the international community," White House spokesman Gordon Johndroe said. "The Iranian regime's continued defiance only further isolates the Iranian people. We urge Iran to suspend its uranium enrichment and reprocessing activities or face further implementation of the existing United Nations Security Council sanctions and the possibility of new sanctions."

Anticipating U.S. condemnation, Iranian Foreign Ministry spokesman Hasan Qashqavi said in Tehran before the report's release that his country expected the IAEA not to bow to U.S. "pressures."

Intelligence received by the IAEA in its investigations, as well as from the U.S. and other agency board member nations, suggest Iran experimented with an undeclared uranium enrichment program that was linked to a missile project and drew up blueprints on refitting missiles to allow them to carry nuclear warheads.

The intelligence also suggested Iran was researching construction of an underground site that apparently could be used to test fire nuclear bombs and ordered "dual use" equipment from abroad that could be part of an atomic weapons program.

Additionally, Iran possesses diagrams showing how to mold uranium metal into the shape of warheads.

U.S. intelligence estimates that Iran conducted such experiments until 2003. Iran rejects such allegations and says documents backing them up are fabricated.

FOX News' James Rosen and The Associated Press contributed to this report.