Foreign envoys toured on Sunday a nuclear plant where Iran is enriching uranium in defiance of UN sanctions, a day after Tehran vowed to push ahead "very strongly" with the sensitive work.
The Islamic republic organised the rare visit in a bid to garner support for its contentious atomic drive ahead of key talks with six world powers in Istanbul next week.
The diplomats, among them representatives of some member states of the UN atomic watchdog, the International Atomic Energy Agency (IAEA), saw Iran's heavy water facility at Arak on Saturday as part of the tour.
Tehran's allies Russia and China snubbed the visit, along with the European Union.
Iran's state broadcaster showed footage of the envoys entering the country's main uranium enrichment facility in the central city of Natanz where the material is being refined despite strong objections from the West.
"What we have done is an unprecedented move... to show 100 percent transparency" about Iran's nuclear programme, Ali Asghar Soltanieh, Iran's envoy to the IAEA, said on state television from the Natanz facility.
The Natanz plant tour comes a day after Iran's atomic chief and acting foreign minister, Ali Akbar Salehi, vowed to push ahead with the enrichment work "very strongly", dismissing reports sanctions and technical problems had hampered the nation's nuclear pursuit.
"The recent sanctions did not create any problems for our nuclear activities," Salehi told a news conference in Arak broadcast live on state television.
"Our nuclear activities are going forward strongly. Our activities, especially in (uranium) enrichment, are also continuing very strongly... The production of enriched uranium is growing."
Salehi's remarks were seen as Tehran's response to comments by US Secretary of State Hillary Clinton last week when she asserted Iran's nuclear programme had been hit by sanctions.
"They have made it much more difficult for Iran to pursue its nuclear ambitions," she said. "Iran has technological problems that have made it slow down its timetable."
Salehi also dismissed reports the programme was hit by the Stuxnet computer virus, which the New York Times said on Saturday was tested by Israel and the United States on Tehran's nuclear installations.
"The Stuxnet issue goes back a year and a half. When they initiated this, they thought we were sleeping... If this was effective, the IAEA, which regularly inspects (Iranian sites), would have reported the slowdown," he said.
In its online edition, the Times quoted intelligence and military experts as saying Israel had tested the effectiveness of the Stuxnet computer worm which apparently shut down a fifth of Iran's nuclear centrifuges in November.
The testing took place at the heavily guarded Dimona complex in Israel's Negev desert housing the Middle East's sole, albeit undeclared nuclear weapons programme, it said.
World powers, led by Washington, want Tehran to halt uranium enrichment, which they suspect is aimed at making weapons. Iran says its nuclear activities are entirely for peaceful purposes.
The dispute will be at the centre of talks between Tehran and six world powers -- Britain, China, France, Russia, the United States and Germany -- in Istanbul on Friday and Saturday.
Iran at the end of October had about 3,200 kilograms (7,000 pounds) of uranium enriched to the 3.5-percent level, according to the IAEA.
Salehi said on Sunday the ongoing tour was not the last one and "there will be other visits in future."
Those participating in the current tour are representatives of the Non-Aligned Movement, the Group of 77, Arab League, Syria, Venezuela and Oman, according to Iran.
The visit has been snubbed by the European Union, and by Iran's key allies on the UN Security Council, Russia and China. Tehran did not invite the United States, Britain, France and Germany.
The last such trip that Tehran arranged for IAEA members dates back to February 2007.