VIENNA (AP) — Talks between the U.N. atomic agency and Iran "have been going around in circles," the organization's head said Monday, acknowledging that attempts to probe alleged secret nuclear weapons work by Tehran have failed and suggesting Iran was to blame.

Yukiya Amano's unusually blunt language in comments to delegates at the opening session of a 35-nation IAEA board reflected his frustration at the lack of progress. Experts of his International Atomic Energy Agency have met Iranian counterparts 10 times since early 2012 in attempts to gain access to sites, documents and persons linked to the IAEA investigation, without tangible headway.

The IAEA had hoped that the meetings would lead to a restart of its investigation that stalled in 2007.

While not putting direct blame on the Islamic Republic, Amano suggested the fault lies with Tehran's perceived refusal to cooperate with IAEA experts looking to investigate the allegations. He appealed to the Islamic Republic to engage with his agency on the probe "with a sense of urgency."

International exasperation with the IAEA-Iran stalemate is compounded by deadlock in repeated attempts by six world powers to persuade Tehran to curb uranium enrichment — a technology that can generate both nuclear power and the core of nuclear weapons. Iran denies any work interest in such arms and insists all of its atomic activities are peaceful. It has been hit by U.N. Security Council and other sanctions for its refusal to halt enrichment.

Amano frequently urges Iran to cooperate. But his departure from softer diplomatic language appeared to be a prelude to a push by the U.S. and its allies for tougher action on Iran that goes beyond the usual critical speech making.

The Security Council first took up Iran's nuclear file on the basis of an IAEA resolution in 2006 suggesting it was not in full compliance with the nuclear nonproliferation treaty, and the agency has since sent several other resolutions to the council.

Western diplomats have said that unless there is progress in the IAEA probe their countries could push for another resolution reporting Tehran to the Security Council. That would add diplomatic pressure on the Islamic Republic and give traction to any attempts by the United States, Britain and France — the Western permanent council members — to lobby for possible new sanctions.

Amano specifically urged Iran to cooperate on IAEA attempts to probe allegations that Tehran worked on explosive triggers for a nuclear blast at the Parchin military complex southeast of Tehran, at a site that the agency suspects may now be undergoing a cleanup.

In comments to reporters Amano mentioned soil removal, asphalt work and "possible dismantling of infrastructures" at Parchin.

Because of such work, he said "it may no longer be possible to find anything even if we have access to the site."