Iran on Thursday signaled it will no longer cooperate with International Atomic Energy Agency experts investigating for signs of nuclear weapons programs, confirming that the probe — launched a year ago with great expectations — was at a dead end.

Coming from Iranian Vice President Gholam Reza Aghazadeh, the announcement compounded international skepticism about denting Tehran's nuclear defiance just five days after Tehran stonewalled demands from six world powers to suspend activities that can produce the fissile core of warheads.

Besides demanding a stop to uranium enrichment — which can create both fuel and the nuclear missile payloads — the international community also has been pressuring Tehran to cooperate with the IAEA in its probe of allegations that Tehran hid attempts to make nuclear arms.

That investigation was launched a year ago under a so-called "work plan" between the Vienna-based agency and Tehran.

Back then, IAEA chief Mohamed ElBaradei hailed it as "a significant step forward" that — if honored by Iran — would fill in the missing pieces of Iran's nuclear jigsaw puzzle; nearly two decades of atomic work, all of it clandestine until revealed by dissidents nearly six years ago. And he brushed aside suggestions that Iran was using the work plan as a smoke screen to deflect attention away from its continued defiance of a U.N. Security Council ban on enrichment.

But the plan ran into trouble just months after it was put into operation. Deadline after deadline was extended because of Iranian foot-dragging. The probe, originally to have been completed late last year, spilled into the first months of 2008, and then beyond.

Iran remains defiant, saying evidence from the U.S. and other board members purportedly backing the allegations was fabricated, and on Thursday Aghazadeh appeared to signal that his country was no longer prepared even to discuss the issue with the Vienna-based IAEA.

Investigating such allegations "is outside the domain of the agency," he said. Any further queries on the issue "will be dealt with in another way," he added, without going into detail.

Britain, one of the main critics of Iran's nuclear activities, was critical.

"We are concerned by reports that Iran is refusing to cooperate with the IAEA on allegations over nuclear weapons," a spokesman for Britain's Foreign Office said on condition of anonymity in line with policy. "The IAEA has raised serious concerns over Iran's activities with a possible military dimension. "If Iran is serious about restoring international confidence in its intentions, it must address these issues."

The IAEA has asked in vain for substantive explanations for what seem to be draft plans to refit missiles with nuclear warheads; explosives tests that could be used to develop a nuclear detonator; military and civilian nuclear links; and a drawing showing how to mold uranium metal into the shape of warheads.

The agency's last Iran report in May reflected ElBaradei's frustration. It said Iran may be withholding information on whether it tried to make nuclear arms, in language described by one senior U.N. official as unique in its direct criticism of Tehran.

Aghazadeh's comments appeared to jibe with those of diplomats familiar with the probe who have told The Associated Press that the IAEA has run into a dead end.

One senior diplomat on Thursday attributed Tehran's intransigence in part to anger about multimedia presentations by IAEA Deputy Director General Olli Heinonen to the agency's 35 board members based on U.S. and other intelligence on the alleged secret weapons work. He — like the others — demanded anonymity because his information was confidential.

The U.S. and allies also link Iran's enrichment program to secret attempts to make nuclear arms and demand a freeze of such activities. But Tehran says its nuclear program is for peaceful purposes — and Iranian President Mahmoud Ahmadinejad on Wednesday again vowed his country will not "retreat one iota" from pursuing it.

First-time U.S. participation on the issue at talks Saturday in Geneva had raised expectations for a compromise under which Iran would temporarily agree to stop expanding its enrichment activities. In exchange, the six world powers — the U.S., Germany, Britain, France, Russia and China — would hold off on adopting new U.N. sanctions against Iran.

But participants at Geneva said Iranian negotiators skirted the freeze issue, despite the presence of U.S. Undersecretary William Burns, which was meant to encourage Tehran to make concessions.

U.S. Secretary of State Condoleezza Rice on Monday accused Iran of not being serious at the Geneva talks. She warned that all six nations were serious about a two-week deadline for Iran to agree to freeze suspect activities and start negotiations or else be hit with a fourth set of U.N. penalties.

But Aghazadeh, who is also head of Iran's Atomic agency, downplayed the international complaints — while evading a direct answer on whether Tehran would give on the issue of an enrichment freeze.

"Both sides are carefully studying the concerns and expectations of both sides," he told reporters.