VIENNA, Austria — Iran has steadfastly blocked a U.N. investigation into allegations it tried to make nuclear arms, and the probe is now deadlocked, the International Atomic Energy Agency said Monday.

The conclusion was contained in an IAEA report released to 35-nation IAEA board and the U.N. Security Council, which has already imposed three sets of sanctions on Iran over its nuclear defiance.

"We've arrived at a gridlock," said a senior U.N. official, describing the document as "a progress report without progress." He demanded anonymity because he was not authorized to discuss the restricted report, made available to The Associated Press.

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

The document said Iran has now amassed a third of the amount of enriched uranium it could reprocess into the material for the fissile core of a nuclear weapon.

The United States and its allies allege Iran wants to develop its uranium enrichment program to make nuclear weapons.

But oil-rich Iran insists it only wants to make nuclear fuel and IAEA oversight and inspections of its known enrichment program has not come up with any evidence that contradicts that.

In Washington, the White House threatened more sanctions if Iran continues to defy the U.N.

"This report shows once again that Iran is refusing to cooperate with the international community," said White House spokesman Gordon Johndroe. "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."

But Ali-Ashgar Soltanieh, Iran's chief delegate to the IAEA, asserted there was nothing for the agency to investigate as far as weapons programs were concerned.

"The Americans did a lousy job with the CIA and associated terrorist groups that work with them," he told the AP, alluding to Iranian claims that the American intelligence pointing to past Iranian weapons programs was forged.

The report confirmed Iran continues to expand its uranium enrichment program in defiance of the three sets of U.N. sanctions.

The document said Iran was now either fully or partially operating nearly 4,000 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 above-ground experimental site at Natanz, a city about 300 miles south of Tehran.

To date, Iran had produced about 1,000 pounds of low enriched uranium suitable for nuclear fuel, said the report. Asked to put that figure into context, U.N. officials said Iran would need three times that amount to be 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.

Iran's refusal to end enrichment has been the main trigger for sanctions and continues to be the overriding concern for the United States and others who suspect the program is aimed at making weapons.

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," said the report.

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.

In Tehran, the official news agency reported Iranian warplanes and air defense units are taking part in ongoing war games. The war games follow Israel's major aerial exercise over the eastern Mediterranean in June, which was widely interpreted as a rehearsal for a possible strike on Iran's nuclear facilities.