Foreign diplomats were set to kick-off on Saturday a two-day tour of Iran's atomic sites at the invitation of Tehran, in a bid to drum up support for its contentious nuclear programme.

The rare tour to Iran's main uranium enrichment facility at Natanz and the heavy water installation in Arak was apparently snubbed by Iran's key allies China and Russia, as well as the European Union.

Iran had invited several envoys to the Vienna-based International Atomic Energy Agency (IAEA) as well as Hungary, the holder of the European Union's rotating presidency, to take part in the tour.

However, the United States, Britain, France and Germany were not invited, according to diplomatic sources.

"The NAM (Non-Aligned Movement) troika, the Group of 77 members, the Arab League, Syria, Venezuela and Oman, are participating in the visit," Iran's envoy to the IAEA, Ali Asghar Soltanieh, told Fars news agency on Saturday.

Russia and China appear to have declined to participate in the visit, as indicated by comments from their capitals.

"(China's) Vienna representative is still at home right now, so it will be difficult for him to go," Beijing's foreign ministry spokesman Hong Lei said on Friday.

Russian Foreign Minister Sergei Lavrov said Thursday that while Iran's invitation "deserved attention," such a trip could not replace IAEA inspections, clearly indicating Moscow would refrain from participating in the trip.

The European Union declined the invitation outright, saying the IAEA "are the people who have to inspect the Iranian nuclear facilities."

Iran's atomic chief and acting foreign minister Ali Akbar Salehi, meanwhile, said Tehran would reveal new achievements on Saturday during the visit to the Arak facility.

"Today, at the heavy water installation in Arak, we will unveil several new nuclear achievements in the field of medicine," Salehi said.

"This achievement will be unveiled in the presence of guests who we have invited from different countries and international organisations."

Iran has pitched the trip as a confidence-building measure.

"No country in the world will show its nuclear installations to others and this is a sign that Iran's nuclear activities are peaceful," Salehi told ISNA on Friday.

Such visits to Iran's atomic sites are infrequent. The last trip that Tehran arranged for members of the IAEA was in February 2007.

The Iranian move came in the run-up to talks with six world powers at the end of next week in Turkey chaired by EU foreign policy chief Catherine Ashton.

Britain, China, France, Russia, the United States and Germany are to meet with Iran for another round of talks on Tehran's atomic programme in Istanbul on January 21-22.

The previous round of talks, after a 14-month hiatus, was held in Geneva on December 6-7.

Western powers suspect Iran wants to use its uranium enrichment activities to build a nuclear bomb. Tehran denies the charge, insisting its programme is a peaceful effort to produce nuclear energy.

Iran maintains it will not talk on its "nuclear dossier" at the Istanbul meeting, while Ashton has categorically said Tehran's nuclear programme would be part of the discussions.

"I'm very clear that we are coming to discuss the nuclear issue and that is what we will do," Ashton told reporters in Istanbul on Thursday.

"Our purpose in meeting is to now look for tangible credible ways to make a move forward," she added.

Iran is under four sets of UN sanctions over its refusal to suspend uranium enrichment, one of the most sensitive parts of its atomic programme because enriched uranium can be used not only to make nuclear fuel, but also the fissile material for a bomb.