Iran said the tour, which China and Russia also snubbed after being discouraged by Western officials, aimed to demonstrate the country's transparency about its atomic program before talks with major powers.

Tehran invited some ambassadors accredited to the International Atomic Energy Agency (IAEA) to visit its nuclear sites. The US, British, French and German envoys were not asked along, while the EU declined its invitation, saying it was the task of UN nuclear inspectors to carry out such visits.

"The EU lost the historic opportunity for further cooperation with Iran and also visiting its peaceful nuclear activities," Ali Asghar Soltanieh, Iran's IAEA envoy, told state TV before the tour.

Analysts said the selective invitation may be aimed at eroding new harmony among the US and European governments on the one hand, and Russia and China on the other in next week's talks with Iran, aimed at curbing its nuclear work.

Alongside the Non-Aligned Movement of developing nations, the group of seven envoys comprised ambassadors from Egypt, Venezuela, Syria, Algeria, Oman and Cuba.

The envoys, who will stay in Iran until Monday, toured the heavy water Arak installation. Later they will also visit the underground Natanz uranium enrichment site where feedstock uranium hexafluoride (UF6) gas is fed into centrifuges. This is to make material to fuel power plants, which could, if greatly enriched, be used for nuclear warheads.

Journalists working for foreign media in Iran were not invited to tour the sites.

The five permanent members of the United Nations Security Council, China, France, Russia, Britain and the United States - which along with Germany are known as P5+1 - are due to hold a second round of talks with Iran on Jan. 21-22 in Istanbul.

Meanwhile, Iran said on Saturday its uranium enrichment program was progressing "very strongly.”

The declaration by Foreign Minister and atomic chief Ali Akbar Salehi’s remarks were a reaction to US Secretary of State Hillary Clinton's recent comment that Iran's nuclear program was hampered by international sanctions.

"The recent sanctions did not create any problems for our nuclear activities," Salehi told a news conference broadcast on state television from the central city of Arak.

"Our nuclear activities are going forward strongly. Our activities, especially in (uranium) enrichment, are also continuing very strongly." World powers, led by Washington, want Tehran to stop the uranium enrichment, which they suspect is aimed at making weapons. Iran denies the charge.