The UN atomic agency Tuesday gave its clearest indication yet that Iran may be developing nuclear weapons despite Tehran's continued insistence that its nuclear programme is entirely peaceful.

Here is a list of its main sites:

- Natanz enrichment facility, central Iran:

Iran revealed the existence of the Natanz plant to the International Atomic Energy Agency (IAEA) in 2002. The site is probably the best known nuclear installation in Iran and is under UN supervision.

There are now more than 8,400 centrifuges at Natanz, including 4,600 active ones.

Underground installations at Natanz can hold up 50,000 centrifuges.

- Qom enrichment facility, central Iran:

Iran revealed the secret uranium enrichment plant in the region of Fordo in September 2009, infuriating the West and prompting the United Nations to strengthen sanctions against Tehran.

The facility was built deep inside a mountain near the Shiite shrine city of Qom, some 150 kilometres (95 miles) southwest of Tehran.

Earlier this year, Tehran announced it would start transferring its 20-percent uranium enrichment activities to the Fordo site and the first centrifuges were installed over the summer.

According to media reports, the facility can take some 3,000 centrifuges.

- Isfahan conversion facility, central Iran:

At this plant, raw mined uranium is transformed into "yellowcake" -- a concentrate of uranium oxides -- which is then transformed into uranium tetrafluoride (UF4) and then into uranium hexafloride (UF6), a feed gas for the actual process of enrichment.

The plant was industrially tested in 2004 upon its completion and is under UN supervision.

- Isfahan nuclear fuel facility, central Iran:

President Mahmoud Ahmadinejad inaugurated the facility on April 9, 2009.

The plant can produce 10 tonnes of nuclear fuel annually to feed the heavy water 40-megawatt Arak reactor and 30 tonnes for light water reactors such as the Bushehr nuclear plant.

The opening of the fuel plant indicated that Iran had mastered the complete nuclear fuel cycle from uranium mining to enrichment, even as world powers urged the Islamic republic to suspend its programme.

In 2010, Tehran also started building a facility there to enrich uranium to 20 percent and it was expected to start churning out the fuel needed for the Tehran research reactor in spring 2012.

- Arak heavy water plant, central Iran:

Work on the Arak heavy water research reactor on the outskirts of the village of Khondab has been delayed and the reactor, whose official function is to produce plutonium for medical research, should not be completed before 2013, according to Iranian officials.

Heavy water reactors do not need enriched uranium fuel in order to function.

The site also includes a production plant for heavy water to be used as coolant and moderator for the reactor.

- Bushehr nuclear plant, south Iran:

Iran's first nuclear power plant, built by Russia, began operating at 40-percent capacity in September, after more than three decades' delay, and will gradually increase to full capacity by March 2012 before it is plugged into the power grid.

The Bushehr project was first launched by the US-backed shah of Iran, Mohammad Reza Pahlavi, in the 1970s using contractors from German company Siemens.

In 1995, Moscow however signed a one-billion-dollar agreement with Iran to complete construction of the plant, three years after Germany declined to resume work citing the threat of proliferation of sensitive nuclear technology.

Russia continues to deliver nuclear fuel for the plant, which remains under IAEA control.

- Tehran nuclear research centre:

Iran obtained the five-megawatt research reactor from the United States before the 1979 Islamic revolution that toppled the US-backed shah. The reactor is under IAEA supervision.

- Saghand uranium oxide extracting mine, south Iran:

The mine has reserves estimated between 3,000-5,000 tonnes of uranium oxide, which can be used to make yellowcake for the Isfahan conversion plant.

- Parchin suspected nuclear site:

Satellite imagery apparently shows an unlisted nuclear installation at the Parchin military site 30 kilometres from Tehran, possibly used for blast tests.

Iran has already produced over 4,500 kilogrammes of uranium enriched at five percent or less, as well as 70 kilogrammes of 20-percent enriched uranium, according to IAEA estimates in September.

Uranium can be used as fuel for nuclear plants when enriched at five percent or less, and as fuel for research reactors at 20 percent. Over 90 percent, it may be used to make a bomb.