A dispute between the US and other members of the UN's nuclear watchdog is holding up a decision on whether the organisation's boss will keep his job.
Mohamed ElBaradei led the UN's inspections in Iraq
Washington does not want Mohammed ElBaradei to serve a third term as director-general of the International Atomic Energy Agency (IAEA).
But the US has offered no other candidate, and Mr ElBaradei is said to enjoy strong and broad support.
A BBC correspondent says the dispute could delay a decision until June.
The IAEA's board of governors has been holding a meeting in Vienna, and was due to confirm who would be its next boss.
'More time needed'
The 35 governors make the decision either by consensus or by a two-thirds majority vote.
The chair of the IAEA board, Canadian ambassador Ingrid Hall, is reported to have told the board that "ElBaradei has strong and broad support but more time is needed to take a decision".
A Western diplomat told the AFP news agency she said the "board is waiting for the United States to accept that there is only one candidate and that he has the board's backing".
The BBC's correspondent in Vienna, Bethany Bell, says the Americans have made it clear they do not want Mr ElBaradei to stand for a third term.
Mr ElBaradei oversaw the UN nuclear inspections during the run-up to the Iraq war, and refused to confirm US claims that Saddam Hussein had weapons of mass destruction.
The US also feels he has not been tough enough against Iran, which Washington says is trying to build nuclear bombs.
The US says IAEA director-generals should be limited to two terms.
Our correspondent says the dispute could mean consultations continue until the board's next meeting, in June.