A group of ambassadors from non-aligned countries has been taken on a tour of a nuclear facility in Iran on what is being billed as a transparency visit.
The tour group was shown yellow cake, or raw uranium
It is the first such trip since the UN imposed limited sanctions on Iran in December for refusing to suspend its uranium enrichment programme.
The UN's chief nuclear inspector is to report on Iran's compliance with the Security Council's demands this month.
Some countries suspect Iran of secretly trying to develop a nuclear weapon.
Iran insists on its right under the Nuclear Non-Proliferation Treaty to enrich uranium on its own soil for what it says is a peaceful nuclear programme.
The touring delegation included six envoys accredited to the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), from Cuba, Malaysia, Egypt, Sudan, Bolivia and Algeria.
Mr Ahmadinejad is to announce important news on the nuclear front
They were taken on a tour, along with journalists, of the nuclear plant near the central Iranian city of Isfahan.
Iran resumed sensitive uranium conversion work there in 2005.
The BBC's Frances Harrison, who was on the tour, says the group was dressed in protective clothing and shown yellow cake, or raw uranium, and then the finished products stored in large cylinders.
The organisers were keen to demonstrate that the IAEA still has surveillance cameras in the plant to observe the large quantities of uranium gas it has produced so far.
Correspondents say the 116-strong Non-Aligned Movement is hoping to reassert the influence it had in the 1960s as a third bloc to the US and Soviet superpowers.
However, they say it is more likely it has been invited to Iran not for its relevance on the world stage but because of its recent endorsement of Iran's stand-off with the US over its uranium enrichment.
UN SANCTIONS ON IRAN
Ban on import and export of nuclear-related material
Assets frozen of 10 companies and 12 individuals
Threat of further non-military sanctions
Tehran is under increasing pressure from the international community over the issue.
Iran insists that it is continuing to co-operate with the IAEA, despite recently barring 38 of the agency's inspectors from entering the country.
On Friday an Iranian official denied reports that Tehran had prevented the IAEA installing surveillance at its uranium enrichment plant at Natanz. The envoys and journalists were not taken to Natanz.
Tehran has frequently said it intends to install 3,000 centrifuges there, stepping up its controversial uranium enrichment programme in defiance of UN Security Council calls for a suspension.
The country's hardline president, Mahmoud Ahmadinejad, said recently that he would soon be announcing some important news on the nuclear front.