"We have been making progress but not with the speed we would like to see," said International Atomic Energy Agency (IAEA) chief Mohamed ElBaradei on a flight taking him to Tehran, where he is to meet with Iranian officials.

The IAEA has given Iran until October 31 to answer questions on Iran's nuclear program. The IAEA began an intensive round of inspections on October 1.

"Iran has been offering us additional information, additional access but not the 100 percent transparency and not the pro-active cooperation I would like to see if we were to be able to get full information we need by the end of the month," ElBaradei said.

Non-compliance by Iran could lead the UN's nuclear watchdog to take the issue to the UN Security Council, which could then impose punishing sanctions on Tehran.

Washington accuses Iran of trying to develop nuclear arms and has branded it part of an "axis of evil", along with the Iraq of Saddam Hussein that it said held weapons of mass destruction, and North Korea, which has claimed it is making atomic bombs.

On Tuesday, Iranian opposition leaders charged in Vienna that Iran was secretly building a uranium-enriching plant for military purposes some 15 kilometres (nine miles) east of the central-western city of Isfahan, allegedly part of a network of small sites meant to guarantee Iran's nuclear program could survive an attack.

The same group, the National Council of Resistance of Iran (NCR), had claimed in August last year that Tehran maintained a secret nuclear site in Natanz, where IAEA inspectors later found traces of enriched uranium though officials said they were due to accidental contamination from imported equipment.

ElBaradei said it was crucial that Iran provide full disclosure on its nuclear program, particularly answering questions about enriching uranium which could be used to make atomic weapons.

"I got an invitation from the Iranian authorities telling me that it would be very important for me to visit at this stage," ElBaradei said.

"I am ready obviously to walk the extra mile if we can bring the issue to closure as soon as possible," he said.

ElBaradei said Iran had not yet indicated which countries it imported equipment from that Tehran claims is the source of highly enriched uranium particles found by IAEA inspectors.

The United States claims that these particles do not come from contaminated equipment from abroad but are in fact proof that Iran is making highly-enriched uranium.

"Our immediate first priority is to understand fully their enrichmenent program, to make sure that we have seen . . . all the nuclear material that exists in the country," ElBaradei said.

"So far based on the information we have not been able to come to that conclusion yet, (things are) pretty much hanging in air," he said.

ElBaradei said the IAEA had "asked to see (both) civilian and military sites" in its investigation and has not had access denied.

Iran wants guarantees that inspections would not comprise its military secrets.

ElBaradei is due to report back on November 7 to his council of governors, the IAEA's executive organ.

Tehran has repeatedly denied it was developing nuclear weapons. On Tuesday it pledged to open talks with the IAEA as of Saturday over demands that it sign an additional protocol to the nuclear Non-Proliferation Treatyallowing for snap inspections of all nuclear sites.

British Foreign Secretary Jack Straw insisted Tuesday he wanted the nuclear standoff resolved calmly but did not rule out possible military action.

ElBaradei met Tuesday night in Vienna with Russian Foreign Minister Igor Ivanov, but no details emerged on their talks.

Russia has been helping Iran build a nuclear plant at Bushehr, in southern Iran, despite US objections. The plant was due to start working next year, but US pressure could push this back to 2005.

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