Violence is the biggest challenge facing women around the world as progress in gender equality is erratic and at times a baffling contradiction, said the top official at U.N. Women ahead of International Women's Day.

Phumzile Mlambo-Ngcuka, executive director of U.N. Women, said despite decades of pushing for equal rights, no one nation could call itself gender equal with countries making advances in some areas yet backsliding in others.

Mlambo-Ngcuka described the global gender pay gap of 24 percent as "the biggest robber" of women. U.N. Women is launching a global coalition to tackle pay inequality during the meeting of the U.N. Commission on the Status of Women next week.

But Mlambo-Ngcuka said the biggest difficulty facing women is violence. One in three women suffer physical or sexual violence during their lifetime, and half of female murder victims are killed by partners or family members, according to U.N. Women.

Some 120 million girls worldwide, roughly one in 10, have experienced forced intercourse or other sexual acts, the group says.

"Obviously, class and geography help some women to survive this issue differently, but everywhere in the world the big issue of violence against women is a reality, whether you are rich or poor, in a developed or developing country," she said.

"Even countries that have the highest indicators on gender equality like Iceland, they still have to confront the issue of violence against women," Mlambo-Ngcuka told Reuters in an interview.

Iceland was shaken in January by the violent death of a 20-year-old woman in Reykjavik. A crewmember from a Greenlandic trawler has been arrested in connection with her death.

Mlambo-Ngcuka, a South African, has since 2013 headed the United Nations' body charged with promoting gender equality and women's empowerment. She was a member of parliament in South Africa's first democratic government and the nation's first female deputy president.

She said global advances in women's rights, from equal pay to boosting women in leadership and lowering rates of violence, were spotty and unequal.

"Countries have constitutions that pride themselves in being democratic, constitutions that define equality, but no one fights for women to be paid equally," she said.

"It is contradictory," she said. "How do you explain to me that Afghanistan has more women in parliament than the Congress in the United States?"

Women hold 28 percent of the seats in the Afghan parliament, compared with 19 percent in the U.S. Congress, according to World Bank 2016 statistics.

Mlambo-Ngcuka cited Colombia, emerging from a 52-year civil war that killed more than 220,000 people and displaced millions, as an example of one nation moving ahead of its neighbors.

Colombia's Congress and rebel insurgents in November reached a peace agreement that pledges land and property rights for women and prosecution of rapes committed by military forces and rebel fighters.

"In that peace agreement, they have made really good strides that address gender equality," Mlambo-Ngcuka said. "Yet there are countries in the neighborhood of Colombia who have not been going through what Colombia has been going through who have not addressed these issues."