VIENNA, Austria — Iran signaled today that it will no longer cooperate with U.N. experts probing for signs of clandestine nuclear weapons work, confirming the investigation is at a dead end a year after it began.

The announcement from the Iranian Vice President, Gholam Reza Aghazadeh, compounded skepticism about denting Tehran’s nuclear defiance, just five days after Tehran stonewalled demands from six world powers that it halt activities capable of producing the fissile core of warheads.

Besides demanding a suspension of uranium enrichment — a process that can create both fuel for nuclear reactors and payloads for atomic bombs — the six powers have been pressing Tehran to cooperate with the International Atomic Energy Agency’s probe.

Iran, which is obligated as a signer of the Nuclear Nonproliferation Treaty not to develop nuclear arms, raised suspicions about its intentions when it admitted in 2002 that it had run a secret atomic weapons program for nearly two decades in violation of its commitment.

The Tehran regime insists it halted such work and is now only trying to produce fuel for nuclear reactors to generate electricity. It agreed on a “work plan” with the Vienna-based IAEA a year ago for U.N. inspectors to look into allegations Iran is still doing weapons work.

At the time, the IAEA chief, Mohamed ElBaradei, hailed it as “a significant step forward” that would fill in the missing pieces of Tehran’s nuclear jigsaw puzzle — if honored by Iran. He brushed aside suggestions Iran was using the deal as a smoke screen to deflect attention from its continued defiance of a U.N. Security Council demand for a halt to uranium enrichment.

The investigation ran into trouble just months after being launched. Deadline after deadline was extended because of Iranian foot-dragging. The probe, originally meant to be completed late last year, spilled into the first months of 2008, and beyond.

Iran remains defiant. It dismisses as fabricated the evidence supplied by America and other members of the IAEA’s governing board purportedly backing allegations that Iranians continue to work on nuclear weapons.

Officials say that among the evidence given to the IAEA are what seem to be Iranian draft plans to refit missiles with nuclear warheads; explosives tests that could be used to develop a nuclear detonator, and a drawing showing how to mold uranium metal into the shape of warheads. There are also questions about links between Iran’s military and civilian nuclear facilities.

Today, Mr. Aghazadeh appeared to signal that his country was no longer prepared even to discuss the issue with the IAEA.

Investigating such allegations “is outside the domain of the agency,” he said after meeting with Mr. ElBaradei. Any further queries on the issue “will be dealt with in another way,” he said, without going into detail.

Britain, one of those suspicious 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,” the British Foreign Office said in a statement. “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 asked in vain for explanations from Iran, and its last report in May said Iran might be withholding information on whether it tried to make nuclear arms. Reflecting Mr. ElBaradei’s frustration, the report used language described by one senior U.N. official as unique in its direct criticism of Tehran.

Mr. Aghazadeh’s comments today appeared to jibe with those of diplomats familiar with the probe who told The Associated Press that the IAEA had run into a dead end.

A senior diplomat today attributed Tehran’s intransigence in part to anger over a multimedia presentation by the IAEA Deputy Director-General, Olli Heinonen, to the agency’s 35 board members based on intelligence about the alleged weapons work. The diplomat, like others, agreed to discuss the matter only if not quoted by name because his information was confidential.

Tehran dismisses the suspicions of America and allies, and President Ahmadinejad yesterday again vowed that his country would not “retreat one iota” from pursuing uranium enrichment.

On Saturday, an American diplomat had participated in talks with Iran held in Geneva, raising expectations that a compromise might be reached under which Iran would agree to temporarily stop expansion of enrichment activities. In exchange, the six world powers — America, 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 the U.S. Undersecretary of State, William Burns.

Secretary of State 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.

Mr. Aghazadeh, who is also head of Iran’s atomic agency, played down the international complaints, but he also evaded a direct answer on whether Tehran would give any ground on an enrichment freeze.

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