UN inspectors have gained rare access to a facility where Iran is developing centrifuges for enriching uranium, and the visit could give them and the world a "better understanding" of Tehran's controversial nuclear programme.

A confidential International Atomic Energy Agency (IAEA) report, obtained by Reuters, admitted that a prolonged IAEA probe into suspected nuclear arms research by Tehran was making little progress, owing to Tehran's lack of cooperation.

However, the report said that the atomic authority had visited an Iranian research and development (R&D) centre for centrifuges on 30 August.

The report, which has not been made public, did not provide details or say where the site was located.

Atomic Bomb

More efficient equipment can help Tehran gather faster material that could be used to make atomic bombs.

Iran has maintained it is refining uranium to fuel a planned network of nuclear power plants. But if enriched to a high concentration, uranium can also be turned into the explosive core of an atomic bomb.

The report's findings "...suggest that Iran is not yet prepared to seriously discuss and explain to the IAEA the past and possibly ongoing military dimensions of its nuclear programme..." the Institute for Science and International Security (ISIS), a leading US think-tank, said in an analysis.

"It is of importance to see the R&D to understand the full scope and status of the programme," former IAEA chief inspector Olli Heinonen, now at Harvard University's Belfer Center for Science and International Affairs, told the news agency earlier.

November Deadline

The IAEA, in July, said it will need an extra €1m ($1.3m, £793,244) to fund the four-month extension of nuclear talks between Iran and the West.

The agency, which was deeply involved in discussions that attempted to give concrete shape to an interim agreement reached last November, has seen its remit and workload extended extensively during the protracted negotiations.

It is thought that the IAEA has almost doubled its workforce. The additional capital will come to be on a voluntary basis and will fund the monitoring capacity the IAEA assumes over Iran's nuclear facilities and its commitment to change.

An initial deadline of 20 July was set for reaching a settlement, but the deadline has now passed, with a four-month extension being agreed.