By Michelle Nichols

UNITED NATIONS (Reuters) - About a third of the United Nations staff are women, but the higher you go, the fewer you will find in the world body, which has had eight male leaders in its 71 years, and next week is expected to approve Portugal's Antonio Guterres as its ninth secretary-general.

As of June 30, 34.8 percent of the 40,131 members of the United Nations secretariat staff were women, while just 17 of the 79 under-secretaries-general, 21.5 percent, were women, according to the latest secretary-general's report on staff demographics.

Now the U.N. Security Council's recommendation that the 193-member General Assembly appoint Guterres to succeed South Korea's Ban Ki-moon has sparked charges by one lobbying group that female contenders were not treated fairly.

"It's a horrendous, tragic, missed opportunity," said Jean Krasno, chair of WomenSG, formed in February 2015 to campaign for a female U.N. chief. "To women around the world it's just horrible; they don't have a voice."

Krasno said female contenders "were somehow held to some different standards, some higher level of standards than the men, and I really think that was unfair." She said her group had written to Security Council foreign ministers, held events with the candidates and tried to mobilize on social media.

Security Council diplomats defended the decision on Thursday, saying Guterres was the best candidate regardless of gender, and pointed to an increase in the number of women contenders as a sign of progress.

"Although it's high time for a woman, we were going to pick the strongest person in this field, and that is exactly what we have done," British U.N. Ambassador Matthew Rycroft said.

A decade ago, only one woman was nominated to be secretary-general, but this year seven of the 13 candidates were women. During the six secret ballots held by the 15-member Security Council to whittle down the field, though, the highest a woman placed was third.

After the council announced it had agreed on Guterres, former candidate and former U.N. climate chief Christiana Figueres of Costa Rica posted on Twitter: "Bittersweet results #NextSG. Bitter: not a woman. Sweet: by far the best man in the race. Congrats Antonio Guterres! We are all with you."

Colombian U.N. Ambassador Maria Emma Mejia leads a U.N. Group of Friends for a Woman Secretary-General, which has some 60 members, and said that while a female candidate had not prevailed, the group would continue to fight for equality.

"I see Guterres, as he described himself from the very beginning, being like a feminist," she said on Thursday, adding that the group, which includes four current Security Council members, would now work to ensure that gender equality remains atop Guterres' agenda.

"Mr. Guterres deserved it; he was the best of the best," Mejia said, adding that the process was fair.

After his selection, former New Zealand Prime Minister and candidate Helen Clark posted on Twitter: "My campaign was worth the effort if it inspired women and girls everywhere to believe they could aim for the top and one day succeed. Thanks all."

(Reporting by Michelle Nichols; Editing by John Walcott and David Gregorio)