The former head of the U.N. refugee agency has emerged as a front-runner in the race to become the next U.N. secretary-general after a secret vote Thursday in the Security Council, a U.N. diplomat tells CNN.

Antonio Guterres, also Portugal's former Prime Minister, is leading a crowded field of 12 candidates, receiving 12 of a potential 15 votes to succeed Ban Ki-moon, a second source says.

Several diplomats said former Slovenian President Danilo Turk was second following a straw poll. The vote was held in council chambers, and no results were officially released. It's the first such vote in 10 years.

A second straw poll may be held next week.

The first diplomat said that Guterres "is the man to beat" but added that the race is not over and opinions could change.

It's possible that a final vote may be months away.

"It's a very big day for the United Nations," Matthew Rycroft, the UK ambassador, said before the first vote. "The United Nations needs a strong secretary-general to lead it in the years ahead."

Guterres was U.N. High Commissioner for Refugees for 10 years.

The straw poll worked well, the two diplomats said, and showed the efficacy of the public grilling by the 193 U.N. member nations in debate events over the last few months.

The secret vote, however, sparked a rare public dispute between the U.N. Security Council and the U.N. General Assembly.

The council notified the General Assembly President Mogens Lykketoft that a vote had taken place but didn't disclose the outcome.

Lykketoft complained in a letter to all countries that the vote results were not passed on.

He said the fact that the vote took place adds 'little value and does not live up to the expectations of the membership."

U.N. spokesman Farhan Haq described the first stage vote as "fascinating and shrouded in mystery."

In Thursday's straw poll, each Security Council ambassador voted on the 12 candidates through individual ballots. The 12 candidates were scored based on three categories -- "encourage" (stay in the race), "discourage" (take a hint) or "no opinion."

There are numerous factors at play in the election of a new U.N. secretary-general. Many ambassadors and U.N. analysts have said the world organization is slipping into stagnation and division, making it more irrelevant.

A major campaign is underway by some diplomats and other groups to elect the first woman as secretary-general, but the early results may not bode well. However, new candidates can still enter the race.

Eight candidates are from Eastern Europe, a region that believes its turn has come to grab the secretary-general post, based on a global rotation pattern. However, that system is not defined in the U.N. Charter.

Samantha Power, U.S. ambassador to the United Nations, told reporters before the meeting that the United States is looking for someone who has great leadership and management skills and who has a commitment to fairness and accountability.

Ban, the current secretary-general, was first elected on January 1, 2007, for a five-year term, which has been renewed once.

Agencies