Guterres, who was head of the UN's refugee agency for 10 years until 2015, emerged as the Security Council's runaway favorite after the latest in a series of straw polls on Wednesday.

Vitaly Churkin, the Russian ambassador to the UN and current Security Council president, told reporters that Guterres would face a vote in the Security Council on Thursday.

"Today, after our sixth straw poll we have a clear favorite, and his name is Antonio Guterres," he said.

Sixth straw poll

The secretary-general of the UN is appointed in a two-stage process. A candidate is first recommended by the Security Council, and must then be approved by the 193-member General Assembly.

Thirteen candidates entered the race, necessitating a series of straw polls held by the Security Council to encourage a thinning of the field and to help to settle on a single candidate.