Mohammed ElBaradei will leave his post in November
The board of the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), has failed to elect a successor to director Mohammed ElBaradei.
After three rounds of inconclusive voting in Vienna on the two candidates vying to succeed him, the 35-member board decided to begin again on Friday.
Japan's Yukiya Amano led South Africa's Abdul Samad Minty but did not get the required two-thirds majority.
Mr ElBaradei, in office since 1997, is due to step down later this year.
"I reckon the chances of deadlock tomorrow are 90%," a European diplomat told Reuters after the closed-door polling on Thursday.
YUKIYA AMANO, LEFT
Aged 62, veteran Japanese diplomat
Specialises in disarmament and nuclear non-proliferation
Seen as close to US on major issues, including Iran
Backed by industrialised nations
ABDUL SAMAD MINTY, RIGHT
Aged 69, veteran South African arms control and non-proliferation negotiator
Former anti-apartheid activist
Seen as more independent of US
Supported by developing nations
Another diplomat said: "A stalemate is more likely than not."
Mr Amano has the support of industrialised countries while Mr Minty is favoured by developing nations.
Some diplomats see Mr Amano as less political than Mr ElBaradei who, they feel, has been too soft on Iran and too ready to speak out on matters outside his mandate.
But developing nations say Mr Minty is a better communicator, with more independence from the US.
If there is still no winner after Friday, fresh nominations will be sought within four weeks. Previous candidates may run again.
Mr ElBaradei, who won the Nobel Peace Prize in 2005, will step down as the IAEA's chief in November.
His three terms in office were marked by rows with the Bush administration over Iraq and Iran, our correspondent says.