But the 67-year-old Nobel Peace Prize winner will be leaving a difficult legacy to his successor, 62-year-old Yukiya Amano of Japan, with the International Atomic Energy Agency no closer to knowing the true nature and extent of Iran's controversial nuclear programme, despite seven years of intensive investigation.

Indeed, the shock revelation in September that Tehran has been concealing a second uranium enrichment site could see the IAEA's 35-member board of governors vote on a resolution condemning Iran for the first time in nearly four years.

Diplomats at the agency have said that the five permanent members of the United Nations Security Council -- Britain, China, France, Russia and the United States -- plus Germany have drawn up a draft resolution to put to the two-day meeting.

It is not yet clear from pre-meeting talks whether the text will actually win the support of the majority of board members, so the so-called P5+1 may finally decide to issue it merely as a statement rather than put it to the vote, one diplomat said.

But the fact that Russia and China are ready to support such a move is seen as a sign of the growing frustration over Iran's stubborn refusal to come clean about its atomic ambitions.

That sense of frustration has been heightened by Tehran's reluctance to sign up to a deal -- brokered by ElBaradei personally -- that would see Russia and France supply the much-needed nuclear fuel for an Iranian research reactor in return for key confidence-building gestures by the Islamic republic.

ElBaradei has stuck his neck out in an effort to bring the deal to fruition, seeing it as a "unique and fleeting opportunity" for helping resolve the long-running standoff between Iran and the international community.

But Tehran has sent mixed signals from the beginning.

Only last week, Iran's Foreign Minister Manouchehr Mottaki categorically ruled out shipping its own stockpile of low-enriched uranium to Russia for further enrichment, one of the key conditions of the deal.

But this week, other top Iranian officials, including vice president and atomic chief Ali Akbar Salehi, said Tehran was ready to send its low-enriched uranium abroad provided there were simultaneous exchange on its own soil of nuclear fuel processed by world powers.

Western countries insist there is no room for further negotiation and that Iran must say either "yes" or "no" to the deal as it was drawn up by ElBaradei in October.

France sees Tehran's refusal to give any official response so far as a rejection of the deal.

But the US is hopeful that Iran could still say "yes" and said it is not yet "at the point" of closing the window on dialogue.

ElBaradei, too, said in Berlin last week that he did "not consider that I have received a final answer" from Tehran.

The IAEA chief is expected to make a statement on the matter in his opening address to the board on Thursday morning.

Other matters on the meeting's agenda include Syria, North Korea and a fuel bank proposal by Russia.

But the main focus will be the agency's latest report on Iran after IAEA inspectors visited the hitherto clandestine enrichment site called Fordo, near the holy city of Qom.

In its report, the watchdog complained that Tehran's delay in disclosing the plant's existence "does not contribute to the building of confidence" and could even be seen as an indication that the Islamic republic might be hiding still other facilities.

Iran said the site was planned as a back-up plant should the Natanz plant be bombed.

But during their visit to the Qom site, IAEA inspectors verified that the plant was built to contain around 3,000 uranium-enriching centrifuges and experts say that would not be sufficient to cover a civilian power programme.

Iran has been enriching uranium for several years at a plant in the central city of Natanz, in defiance of three sets of UN sanctions. That plant currently has around 8,000 centrifuges installed.