Iran must start answering questions about alleged nuclear weapons work immediately, UN nuclear chief Yukiya Amano said in Vienna on Monday, dpa reported.
The International Atomic Energy Agency (IAEA) director general and his team have been pushing Iran since January to agree to a probe of several suspicious research and developent projects.
"This is frustrating because, without Iran's full engagement, we will not be able to start the process to resolve all outstanding issues, including those concerning possible military dimensions to its nuclear programme," Amano said.
He was speaking at annual board meeting of the IAEA.
The IAEA reported last month that Iran has doubled the number of centrifuges installed at its underground uranium enrichment plant at Fordow, in defiance of UN Security Council demands to halt such work.
The report also said Iran may be removing traces of nuclear warhead experiments at the Parchin military complex.
"We consider it essential for Iran to engage with us without further delay on the substance of our concerns," Amano said.
Tehran's leaders deny that they plan to build atomic weapons and insist that they are only interested in nuclear energy and peaceful scientific applications.
Western countries represented in the 35-country IAEA board would like to censure Iran with a resolution in the week-long meeting, according to Western diplomats.
However, Russia's and China might not back such a step and might only sign on to a joint statement of the five permanent UN Security Council members and Germany, the diplomats said.
The IAEA meeting was being held against the backdrop of international concerns that Israel could attack Iran's nuclear installations if diplomacy fails.