VIENNA, Austria (Reuters) -- Iran's persistent failure to clear up concerns about its nuclear activities after concealing them for almost 20 years sets it apart from all other nations, the U.N. atomic watchdog chief said on Monday.

Six world powers are now negotiating on widening sanctions against Iran for pressing ahead with its program to enrich uranium, a possible route to building atomic bombs, and ignoring a February 21 U.N. Security Council deadline for it to stop.

"Iran's verification case is sui generis (one of a kind)," Mohamed ElBaradei, director of the International Atomic Energy Agency, said in opening remarks to a gathering of the IAEA's 35-nation board of governors.

"Unlike other verification cases, the IAEA's confidence about the nature of Iran's program has been shaken because of two decades of undeclared activities (until 2003)," he said.

"This confidence will only be restored when Iran takes the long overdue decision to explain and answer all the agency's questions and concerns about its past nuclear activities in an open and transparent manner."

Iran rejects Western suspicions that it is trying to master nuclear bomb technology under the cover of a civilian atomic energy program, saying it only wants to generate electricity.

Tehran has also complained of unfair treatment, noting the IAEA has found no hard evidence of covert bomb making efforts. It has characterized sanctions as a U.S.-led campaign to stunt its economic development and topple its government.

"We have not seen concrete proof of diversion of nuclear material, nor the industrial capacity to produce weapons-usable nuclear material, which is an important consideration in assessing the risk," said ElBaradei.

"But quite a few uncertainties remain about experiments, procurements and other activities ..." Iran's IAEA envoy, Ali Asghar Soltanieh, said it would never cede "its inalienable right" to enrichment but was prepared to resolve outstanding issues "if our nuclear dossier is returned (by the Security Council) to the IAEA where it belongs."

Cuts in nuclear aid
At the meeting likely to run some four days, governors were expected to approve cuts to 22 of 55 IAEA technical aid projects in Iran. This would uphold a December U.N. ban on giving Iran technology and know-how of use in making atomic fuel.

A February 22 IAEA report said Iran was installing cascades, or networks, of 164 centrifuges each in its underground uranium enrichment plant in a bid to graduate from research-level refinement of nuclear fuel to a basis for "industrial-scale" production, with some 3,000 centrifuges due to be set up by May.

But ElBaradei said Iran apparently had not begun pumping uranium gas into cascades in the vast Natanz bunker complex, as it said it would start doing by the end of February.

"While there is concern about Iran's future intentions, the situation today is still very much R&D (research and development) activities," he told reporters.

ElBaradei praised an apparent nuclear climb-down by North Korea, whose own confrontation with the world eased when it agreed on February 13 to dismantle its nuclear arms program and readmit IAEA inspectors expelled four years ago.

"I welcome the Beijing agreement, and the invitation to visit North Korea, as positive steps toward the denuclearization of the Korean peninsula and towards the normalization of North Korea's relationship with the agency," he told the board.

ElBaradei goes to Pyongyang on March 13 to work out details of the nuclear shutdown and redeploying inspectors by mid-April to ensure the secretive Stalinist state upholds the pact.

"This process has to be completed within 60 days so we have a short time span (to achieve it) ...," he said.