The stern warning came after the Iranian opposition gave details on a secret nuclear installation where it claims the regime is enriching uranium with a view to producing atomic weapons.

IAEA spokeswoman Melissa Fleming told AFP: "There are two phases to our work. The first phase involves Iran providing all the information to us on unresolved questions no later than October 31."

Fleming said the second phase entailed verification of the information provided by Tehran.

The nuclear watchdog in mid-September gave Iran until October 31 to allay concerns that it may be developing nuclear weapons, warning that it could refer the issue to the UN Security Council, which could in turn slap sanctions on Tehran.

ElBaradei, who is the IAEA's secretary general, was due to arrive in Tehran for a visit on Thursday on invitation of the Islamic republic.

Confirming the visit Monday, the IAEA said: "As agreed with Iranian officials, the purpose of Dr ElBaradei's visit would be for Iran to provide the IAEA during that visit with all the remaining information required to clarify important questions that are still outstanding about Iran's nuclear programmes."

Inspectors' concerns focus in particular on traces of highly enriched uranium found on two samples they took from a nuclear site in the central town of Natanz.

The same opposition leaders who blew the whistle on Natanz in August 2002 alleged in Vienna on Tuesday that Iran was secretly building a uranium-enriching plant.

The site is being constructed some 15 kilometres (10 miles) east of the central-western city of Isfahan and "has been built to test centrifuges that enrich uranium," said Firouz Mahvi, spokesman in Austria of the National Council of Resistance of Iran.

Mahvi said the 150,000 hectare (370,500 acre) site known as Isfahan's "fuel research and production centre" included other installations and "120 to 180 centrifuges are planned to be installed at this facility."

The allegations were vigorously denied by Iran's representative to the IAEA, Ali Akbar Salehi, but Fleming said the IAEA was listening to what the Iranian opposition had to say.

"We're taking good note of these elements that have gotten to us," she said.

ElBaradei was Tuesday evening due to meet Russian Foreign Minister Igor Ivanov before leaving for Tehran on Wednesday.

Russia is helping Iran build a nuclear plant at Bushehr, in southern Iran, and operations could be delayed under pressure of the United States which accuses Tehran of using its atomic power programme as a cover to make nuclear weapons.

Ivanov in Geneva Tuesday reiterated Russia's desire for a peaceful solution to the standoff and urged Tehran to sign a protocol authorising more intrusive IAEA inspections.

"We believe it is necessary that Iran signs up to the additional protocol," he added.

Tehran has stalled on the additional protocol to the nuclear Non-Proliferation Treaty but a top official signalled on the eve of ElBaradei's visit that it was ready to open talks on the matter on Saturday.

And President Mohammad Khatami told reporters in Tehran: "We are very benevolent people and we do not want to create problems. We have declared that we will cooperate with all our strength with the IAEA."

WAR.WIRE