VIENNA, Austria — Iran signaled yesterday 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 Vice President Gholam Aghazadeh of Iran 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 nuclear 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.