The head of the U.N. nuclear agency made an unusually strong demand Monday for Iran to cooperate with an investigation into suspected secret work on nuclear weapons, expressing his frustration with the lack of headway in the probe.

Yukiya Amano also warned that months of delays were hurting attempts by his International Atomic Energy Agency to look into concerns at one particular site — the Parchin military complex southeast of Tehran, where the IAEA thinks Iran may have experimented with blast tests used to set off a nuclear charge.

"We need to stop going around in circles," Amano said of months of fruitless IAEA-Iran talks aimed at gaining access to the Parchin site and make inroads into agency probes of other alleged nuclear arms experiments. "This is frustrating."

Asked why there was no progress on the Parchin probe, Amano told reporters: "You'd better ask Iran."

Iran denies any interest in nuclear arms. It dismisses Western accusations that it is enriching uranium to be able to make the fissile core of nuclear arms, saying the process is meant only to produce reactor fuel. And it rejects allegations that it experimented with components of a nuclear weapons program and denied IAEA experts access to sites, documents and officials linked to the work.

Tensions are growing as time drags on, with Israeli officials using the lack of progress in both IAEA and international diplomatic efforts to engage Iran in their argument that there is a need to bomb Iran's nuclear installations.

Amano's choice of words on the first day of a 35-nation IAEA board meeting Monday appeared unparalleled in reflecting irritation with what agency officials describe privately as foot-dragging by Iran to gain time in expanding some parts of its atomic program and remove traces of other activities.

Board members at a closed IAEA meeting last week were shown satellite images of Parchin allegedly documenting months of clean-up at the site, capped last month by what IAEA officials said were photos showing that tarps had been thrown over buildings where the alleged blast experiments took place.

"We have observed extensive activities at the site and we are concerned that these activities will significantly hamper the verification activities of the agency," Amano told reporters, alluding to the satellite images.

But while expressing concern that delays would reduce the chance of finding evidence of any weapons-related work, he said that — if granted access — "we have powerful tools to find and trace indicators of these activities."