Foreign diplomats were on Sunday to tour a plant where Iran is enriching uranium in defiance of UN sanctions, a day after Tehran vowed to push ahead "very strongly" with the controversial work.

The Islamic republic already threw open two of its atomic sites to the diplomats on Saturday, in a rare move to garner support for its contentious atomic drive ahead of key talks with six world powers in Istanbul this week.

The diplomats, among them representatives of some member states of the UN atomic watchdog, the International Atomic Energy Agency (IAEA), visited the heavy water facility at Arak.

On Sunday, they were to tour Iran's main uranium enrichment facility in the central city of Natanz where the material is being refined despite objections from the West.

Iran's atomic chief and acting foreign minister, Ali Akbar Salehi, said his country will push ahead with the enrichment work "very strongly," dismissing reports that sanctions and technical problems had hampered the nuclear drive.

"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."

He said on Sunday that the latest tours were not the last and "there will be other visits in future."

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-Saturday.

Iran at the end of October had around 3,200 kilograms (7,000 pounds) of uranium enriched to 3.5 percent level, according to the IAEA.

Salehi's remarks were seen as Tehran's response to comments by US Secretary of State Hillary Clinton in Abu Dhabi 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 that the nuclear 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.

Those participating in the current tour of Iranian atomic sites are representatives of the Non-Aligned Movement, the Group of 77, Arab League, Syria, Venezuela and Oman, according to Iran.

The tour 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.

Iranian foreign ministry spokesman Ramin Mehmanparast hit back at the European Union and some "representatives of the five-plus-one" group which is to attend the Istanbul talks for having turned down the tour.

"Apparently this tour did not bear the results they wanted," he said in a veiled reference to Russia and China who were among the invitees on the rare visit of Iranian nuclear sites.

The last such trip that Tehran arranged for IAEA members dates back four years to February 2007.