LONDON - One girl under 15 is married every seven seconds, according to a report by Save the Children released on Tuesday, with girls as young as 10 married off - often to much older men - in countries including Afghanistan, Yemen, India and Somalia.

Early marriage not only deprives girls of education and opportunities but increases the risk of death or childbirth injuries if they have babies before their bodies are ready.

"Child marriage starts a cycle of disadvantage that denies girls the most basic rights to learn, develop and be children," said Save the Children International CEO Helle Thorning-Schmidt.

"Girls who marry too early often can't attend school, and are more likely to face domestic violence, abuse and rape. They fall pregnant and are exposed to STIs including HIV."

The report ranks countries from the best to the worst in which to be a girl, based on child marriage, schooling, teen pregnancy, maternal deaths and number of female lawmakers.

Niger, Chad, Central African Republic, Mali and Somalia were ranked at the bottom of the index.

Researchers say conflict, poverty and humanitarian crises are major factors that leave girls exposed to underage marriage.

The shutting down of schools in the wake of the Ebola outbreak led to an estimated 14,000 teen pregnancies in Sierra Leone during the Ebola outbreak, Save the Children said.

The global charity gave the example of Sahar, who did not want to give her real name, a 14-year-old Syrian refugee in Lebanon. Married to a 20-year-old man, aged just 13, Sahar is now two months pregnant.

"The wedding day, I was imagining it would be a great day but it wasn't. It was all misery. It was full of sadness," Save the Children quoted Sahar as saying.

"I feel really blessed that I am having a baby. But I am a child raising a child."

The UN's children's agency UNICEF estimates the number of women married in childhood will grow from 700 million today to around 950 million by 2030.

Save the Children's report coincides with International Day of the Girl on Tuesday, which was set up by the United Nations in 2011 to recognise the rights of the 1.1 billion girls around the world and the challenges they face.

Meanwhile, British actress Emma Watson condemned child marriage during a visit to Malawi, calling on authorities across Africa to end the practice that can put young lives as risk as well as stopping girls get an education and a better future.

Watson, 26, a goodwill ambassador for UN Women, was visiting the southeastern African nation a day ahead of the International Day of the Girl Child.

In 2015, Malawi passed a law to raise the minimum age of marriage to 18 with UN Women working with traditional chiefs to change local practices.

"Meeting with young girls, who like many in their country, are struggling with poverty and were pressured into early marriage ... made me realize just how important it is for women to be able to make their own choices," Watson said in a statement.

"It's so encouraging to see how such a harmful practice can be stopped when communities work together," added the actress who rose to fame as Hermione Granger in the Harry Potter series.

Even though Malawi has now made child marriage illegal, campaigners have voiced concerns that marriages regulated by custom can occur informally.

During her visit, Watson met with traditional leaders who have championed the annulment of child marriages in their communities.

Worldwide, 15 million girls are married as children every year, according to UK-based campaign group Girls Not Brides.

Campaigners say child marriage deprives girls of an education, increases the risk of domestic violence, death or serious injuries if they have children before their bodies are ready.

Yet the practice persists because some societies view girls as a financial burden while others believe a girl should marry as early as possible to maximize her fertility.

In July, Gambia became the 13th nation in Africa to join the African Union's campaign to end child marriage, which aims to raise awareness of the risks of the practice.

The International Day of the Girl Child, on Oct. 11, is a day declared by the United Nations to promote girls' human rights.