The UN atomic watchdog met on Thursday to elect a successor to Mohamed ElBaradei who is stepping down after 12 years as head of the International Atomic Energy Agency.
The IAEA's 35-member board of governors was to begin a special closed-door session at 10:30 am (0830 GMT) to choose between candidates from Japan, South Africa and Spain.
Initially, there were five candidates for the influential post. But Belgian Jean-Pol Poncelet and Slovenian candidate Ernest Petric pulled out of the race after getting indications that they had no support.
In a straw poll on June 9, Japanese ambassador to the IAEA, Yukiya Amano, held the lead, winning 20 votes out of 35, followed by South African ambassador Abdul Samad Minty with 11 votes.
Spanish candidate Luis Echavarri took four votes, while both Poncelet and Petric won no votes.
At Thursday's meeting, the board would seek to narrow the field down to two with a first vote.
Then it would hold a formal vote to choose the winner. It was not clear whether the formal vote would be held on Thursday or on Friday; diplomats close to the IAEA said.
But if Echavarri fails to win more votes, it could turn into a re-run of the first formal vote back in March between Amano and Minty.
At that time, neither was able to secure the necessary two-thirds majority.
Amano, 62, had been perceived to be the preferred candidate of the West and Minty, 67, the favourite of developing nations with neither able to bridge the gap.
ElBaradei is stepping down in November after three four-year terms as IAEA director general. The agency is playing a key role in international controversies over nuclear programmes in Iran, North Korea and Syria.
The IAEA recently released two new reports, which showed it had made little progress in its probes into the alleged illicit nuclear activities in Iran and Syria.
Iran is defying the UN Security Council and amassing low-enriched uranium, which the United States and its allies fear could play a crucial role in building a nuclear weapon. In a separate report on Syria, the agency said its inspectors found uranium particles at a research reactor near Damascus that would not normally be expected there and had asked Syria to explain.
International efforts to negotiate an end to North Korea's nuclear weapons programmes have virtually collapsed and the Stalinist state conducted what it claimed was a nuclear test last month.
ElBaradei's successor will also have to persuade IAEA member countries to contribute more money to the agency.
During his term in office, ElBaradei, who won the Nobel Peace Prize in 2005 for his work at the IAEA, has been criticized for his comments and accused of politicizing the agency.