Iran has allowed top UN nuclear monitors to visit an advanced centrifuge development site for the first time in a gesture of transparency about its disputed atomic drive, diplomats familiar with the matter said.

One of the diplomats, close to the IAEA, said the IAEA was nearing the end of an inquiry into Iran’s nuclear activity and cited concern a new big power move to increase sanctions on Tehran could hurt the process.

Six world powers agreed in Berlin on Tuesday to the outline of a new UN sanctions resolution although diplomats said the draft lacked punitive trade measures Washington had sought.

The West suspects Iran, which hid efforts to enrich uranium from the IAEA until 2003, suspect Iran’s declared quest for nuclear-generated energy is a front for bombmaking.

Iran denies this and has defied UN resolutions, demanding a nuclear halt, instead expanding an underground enrichment plant.

After a rare Tehran visit by IAEA Director Mohamed ElBaradei on January 11-12, the agency said Iran agreed to settle remaining questions in the long stalled inquiry within four weeks and also handed over some information about efforts to develop “a new generation” of centrifuges able to refine uranium much faster.

On Wednesday, diplomats familiar with IAEA-Iran relations said ElBaradei and his safeguards chief, Olli Heinonen, also visited a Tehran site where a centrifuge to replace Iran’s current outmoded, breakdown-prone model is being developed.

“This was a research and development lab for their new design of P-2 centrifuge that they were able to see,” the first diplomat said, in what was the first such visit since Iranian President Mahmoud Ahmadinejad disclosed the activity in 2006.