UN inspectors have gained rare access to an Iranian nuclear facility, giving them a "better understanding" of Tehran's disputed programme, it has been reported.

They observed a plant where centrifuges for enriching uranium were developed as part of a transparency deal but acknowledged that Iran remains resistant to an International Atomic Energy Agency (IAEA) investigation.

According to a confidential IAEA report obtained by Reuters, inspectors visited a research and development centre for centrifuges on 30 August. It gave neither details nor the location of the site. But such access could be crucial to helping the agency determine how far along Iran might be in developing more modern models of the machines.

Iran's efforts to replace the breakdown-prone, 1970s vintage IR-1 centrifuge it is now operating at its Natanz and Fordow enrichment plants are closely watched by the west since success could lead to more efficient equipment enabling the country to amass material that could be used for atomic bombs more quickly.

Iran says it is refining uranium to fuel a planned network of nuclear power plants. If enriched to a high concentration of the fissile U-235 isotope, uranium can also be turned into the explosive core of an atomic bomb.

"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 Centre for Science and International Affairs, told Reuters.

Apart from an R&D centrifuge plant at Natanz to which the IAEA already has regular access, Iran has a facility called Kalaye Electric in Tehran that the UN agency has seldom visited. Heinonen said R&D work was also done at other locations.

It is not clear when the IAEA was last able to examine an R&D site but such visits are believed to be rare. One was made in 2011 and another in 2008, though it is not known whether they were to Kalaye or some other location.

Iran denies allegations its enrichment programme is part of a covert bid to develop the capability to make nuclear weapons, but western states have imposed economic sanctions to make it scale back its atomic activities.

Friday's IAEA report, which has not yet been made public, said Iran missed a deadline on 25 August to address suspicions about activities in the past that could be relevant to any attempt to assemble nuclear bombs.

It said Iran had implemented just three of five nuclear transparency steps that it had agreed to carry out by last month's target date.

The two measures it had failed to implement concerned providing information about alleged research that is part of the IAEA's long-running investigation into what it calls the "possible military dimensions" of Iran's nuclear programme.