A defiant Iran said on Saturday its controversial uranium enrichment programme was progressing "very strongly," as it opened the doors of two of its nuclear sites to foreign diplomats.

The declaration by foreign minister and atomic chief Ali Akbar Salehi came just days ahead of a high-profile meeting between Tehran and six world powers over the Islamic republic's nuclear programme.

Salehi's remarks were also a reaction to a comment by US Secretary of State Hillary Clinton that Iran's nuclear programme was being 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 ... The production of enriched uranium is growing."

World powers, led by Washington, want Tehran to stop the uranium enrichment, which they suspect is aimed at making weapons. Iran denies the charge.

To try to ascertain whether Iran is making nuclear weapons from the enriched uranium, Britain, China, France, Russia, the United States and Germany are to sit down with Tehran on January 21 and 22 in Istanbul.

Iran currently has around 3,200 kilogrammes (7,000 pounds) of uranium enriched to 3.5 percent level, and nearly 40 kilogrammes of the material refined to 20 percent level.

On Monday, Clinton said in Abu Dhabi that Iran's nuclear programme was hit by a series of sanctions imposed since June 2009 by the UN Security Council and individual countries.

"They have made it much more difficult for Iran to pursue its nuclear ambitions," she said. "Iran has technological problems that has made it slow down its timetable."

In the past, Clinton has said sanctions had begun to hurt Iran economically, forcing it to return to negotiations, but she had not previously said the nuclear programme was being affected.

Salehi said the two-day tour by foreign diplomats of its enrichment plants at Natanz and the heavy water facility in Arak that began on Saturday was a sign of Tehran's peaceful nuclear goals.

"This visit is an evidence that Iran has nothing to hide. Iran is probably the only country to open its (atomic) sites to visitors," said Salehi, who spearheads the nuclear programme.

"We have a just course. What we say is truth and nothing but the truth."

The visiting diplomats went on Saturday to the still-unfinished Arak facility, where Salehi also announced that Iran has begun making nuclear compounds manufactured from the plant's heavy water.

State television said these could be used in the fields of medicine, including for treating bone cancer, and agriculture.

Iran had invited several representatives of member states of the International Atomic Energy Agency (IAEA) to visit the two plants.

Those participating are representatives of the Non-Aligned Movement troika, the Group of 77, the Arab League, Syria, Venezuela and Oman, according to the Iranian envoy to the IAEA, Ali Asghar Soltanieh.

The official IRNA news agency and state television quoted an unnamed Algerian envoy who represents the Group of 77 as saying: "This invitation has created the possibility of continuing talks and more cooperation between Iran and the IAEA."

The tour has been snubbed by the European Union, and by Iran's key allies Russia and China. Iran did not invite the United States, Britain, France and Germany.

"China and Russia have their own political considerations ... Our goal is not to divide the five-plus-one, but is to show our progress and transparency," Salehi said, referring to the six world powers.

Earlier on Saturday, Soltanieh said China was represented at the tour but did not reveal at what level.

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

Soltanieh said the visit was "for agency (IAEA) ambassadors and not linked to agency inspections." He added that the European Union had missed a "historic opportunity" but that "we respect their decision."

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

burs-hif/hkb