Female genital mutilation (FGM) is exacting a "crippling" economic toll on many countries, the United Nations said on Thursday as it launched a tool to help them calculate the cost of treating girls and women harmed by the practice.

An estimated 200 million girls and women worldwide have undergone FGM, which causes a host of mental and physical health problems including hemorrhaging, chronic infections, cysts and life-threatening childbirth complications.

The World Health Organization (WHO) calculated it would cost $1.4 billion a year to treat all resulting medical needs.

"FGM is not only a catastrophic abuse of human rights that significantly harms ... millions of girls and women, it is also a drain on a country's vital economic resources," said Ian Askew, WHO's head of sexual and reproductive health.

The ancient ritual - mostly carried out between infancy and 15 years - involves the partial or total removal of external genitalia. In some cases the vaginal opening is also sewn up.

The new calculator tool, launched on International Day of Zero Tolerance for FGM, covers 27 countries, mostly in Africa.

The cost in Egypt would be about $876,825,000 and Sudan $274,765,000. In some countries the costs would amount to 30% of their yearly health expenditure, demonstrating the clear economic benefits of ending FGM, WHO said.

But global anti-FGM charity 28 Too Many said the health costs were a "drop in the ocean" compared to the wider costs for society and the economy.

"Girls who undergo FGM are often married off young, limiting their education and prospects," executive director Ann-Marie Wilson told the Thomson Reuters Foundation.

"This entrenches poverty in communities and seriously holds back countries' economic development."

WHO scientist Christina Pallitto, who worked on the tool, said the long-term impacts of infection and pain could also affect girls' school attendance and work opportunities.

"All of this radically undermines the ability of women and girls to meet their potential," she added.

World leaders have pledged to end FGM by 2030, but U.N. data published on Thursday showed rates in some countries were the same as 30 years ago, including in Somalia where the practice remains almost universal.

WHO estimated that the health cost of FGM would soar by 50% by 2050 if no action is taken, as populations grow and more girls are cut.