VIENNA: Iran was widely expected to deliver Thursday its response to a UN-brokered proposal regarding the supply of much-needed fuel for a nuclear research reactor in Tehran.

With a deal seen as crucial to resolving the long-running stand-off over the Islamic Republic's atomic programme, Tehran's response was due on the same day that UN experts were scheduled to return from inspecting a hitherto undeclared nuclear site in Iran.

According to a report by the Mehr news agency, Iran will accept the overall framework of a proposal drawn up by the International Atomic Energy Agency last week under which it will hand over much of its stockpile of low-enriched uranium (LEU) to Russia for further processing.

Such a move is aimed at appeasing western fears the material could be used to make a bomb.

But Tehran will nevertheless propose some "modifications" to the arrangement, Mehr reported.

Iran's envoy to the IAEA, Ali Asghar Soltanieh, is expected to hand over the regime's response to watchdog chief Mohamed ElBaradei some time during the day.

The exact time of the meeting was not yet known. But hours earlier, IAEA inspectors were scheduled to arrive back in Vienna after their first-ever visit to a second Iranian uranium enrichment plant, the existence of which was only revealed last month.

During a three-day visit, the four-member team of experts inspected the site, which is being built inside a mountain near the Shiite holy city of Qom.

The disclosure of its existence on September 21 triggered widespread outrage in the West, which suspects Iran is enriching uranium with an ultimate goal of using it to make atomic weapons.

Tehran strongly denies the charge.

Iran has already been enriching uranium -- the most controversial aspect of its nuclear project -- for several years at another plant in the central city of Natanz, in defiance of three sets of UN sanctions.

Enriched uranium produces fuel for civilian reactors, but in highly extended form can also make the fissile core of an atomic bomb.

The IAEA experts had wanted to compare the information of the plant provided by Iran with what they actually found at the facility.

They also wanted to take environmental samples from around the plant to analyse if any radioactive material has been moved into the facility.

Iranian officials have previously said that no radioactive materials have been put inside the plant as it is still under construction.

With regard to the uranium enrichment deal put forward by the IAEA and already approved by western countries, Tehran will export to Russia more than 1,200 kilos (2,640 pounds) of its 3.5 percent LEU for refining up to 20 percent purity to fuel the Tehran reactor that makes medical isotopes.

France would then fashion the material into the fuel rods for the reactor.

Iran had originally been expected to respond to the deal by last Friday but delayed it amid conflicting views on it from its senior officials who are largely of the opinion that Tehran must transport its LEU in batches rather than all at once.