The UN nuclear agency made no progress in a year-long push to find out if Iran worked on developing an atomic bomb, its chief said on Thursday, calling for urgent efforts to end Tehran's standoff with the West.

Yukiya Amano said he would not give up seeking to end what Western diplomats describe as Iranian stonewalling of the agency's investigation into possible military dimensions to the Islamic state's nuclear programme.

UN inspectors will meet Iranian officials for a new round of talks in Tehran next month to seek an agreement to allow the agency to resume its inquiry, after several meetings since January failed to achieve a breakthrough.

"Now is the time for all of us to work with a sense of urgency and seize the opportunity for a diplomatic solution," Amano, director general of the International Atomic Energy Agency (IAEA), told its 35-nation governing board.

Iran rejects suspicions it is on a covert quest for atomic bomb capability.

But its refusal to curb nuclear work with both civilian and military applications, and its lack of openness with the IAEA, have drawn tough Western sanctions and a threat of pre-emptive military strikes by Israel.

A year ago, the IAEA published a report with a trove of intelligence indicating past, and some possibly continuing, research in Iran that could be relevant for nuclear weapons.

They included suspected high-explosive experiments at the Parchin military site southeast of Tehran, and possible work on designing a device to produce a burst of neutrons for setting off a fission chain reaction.

The UN agency, whose mission is preventing the spread of nuclear weapons in the world, has since tried to gain access to Iranian sites, officials and documents it says it needs for the inquiry.

"I am unable to report any progress on clarifying the issues relating to possible military dimensions to Iran's nuclear programme," Amano said, in a frank statement to the agency's policy-making body in Vienna.

"WINDOW OF OPPORTUNITY"

He expressed concern that "extensive activities" at Parchin - an allusion to suspected clean-up work there - would seriously undermine the agency's investigation, if and when it was allowed to visit the sprawling facility.

"Satellite imagery shows that extensive activities, including the removal and replacement of considerable quantities of earth, have taken place at this location," Amano said, according to a copy of his speech to the closed-door meeting.

Tehran says Parchin is a conventional military facility and has dismissed Western allegations that it is trying to remove evidence of any illicit nuclear-related experiments.

It says it must first reach a broader agreement with the IAEA on how the agency should conduct its investigation before it possibly allows inspectors to visit Parchin.

The IAEA's talks with Iran are separate from - but closely linked to - efforts by six world powers to resolve the decade-long nuclear dispute with Iran that has raised fears of a new war in the Middle East.

Diplomacy between Iran and the powers - the United States, China, Russia, France, Germany, and Britain - has been deadlocked since a June meeting that ended without success.

Both sides now say they want to resume talks soon, after the re-election of US President Barack Obama, and diplomats expect a new meeting in Istanbul in December or January.

"We all recognise that there is a window of opportunity and that window is not very big and it is not going to be open for very long," one envoy in Vienna said.

Iran has faced a tightening of Western trade sanctions which the United States and its allies hope will force Iran to curb its nuclear programme.

Tehran has so far showed no sign of doing so, with its atomic energy chief this week saying Iran will go on refining uranium "with intensity". Enriched uranium can fuel nuclear power plants but also provide material for bombs. – Reuters