The auction of a South Sudanese girl for marriage on Facebook could spur other families to use social media to demand bigger dowries for their daughters, activists have claimed.

Plan International has urged authorities and the social networking giant to take much tougher action following the incident in the wartorn African country.

A picture of the 17-year-old, who is from South Sudan's Eastern Lakes state, was posted on Facebook on October 25.

The post said five men were participating in the auction for her dowry.

The highest bidder, a wealthy businessman, reportedly gave the girl's father over 500 cows, three luxury cars and $10,000 as dowry - and the teenager's marriage took place on November 3, according to major girls' rights charity Plan International.

Taban Abel, information minister in Eastern Lakes in the centre of the war-torn nation, said the girl had since gone into hiding in the capital, Juba, but did not offer any more details.

Pictures of the girl in a white wedding gown were shared on various social media sites. Some reports referred to her as a "virgin bride" or "the most expensive woman in South Sudan".

"That a girl could be sold for marriage on the world's biggest social networking site ... is beyond belief," said George Otim, country director of Plan International South Sudan.

Facebook told the Thomson Reuters Foundation it removed the post and the user from its platform for violating its community standards last Friday, when it was made aware of the material.

"Any form of human trafficking whether posts, pages, ads or groups that co-ordinate this activity are not allowed on Facebook," a spokesman said.

Campaigners also called on the Sudanese authorities to take action against those involved in the bidding as some were reportedly state officials, according to Plan and local media.

Despite the legal age of marriage being 18 years old, more than 50pc of South Sudanese girls are wed before their 18th birthday, says the United Nations children's agency UNICEF.

High levels of poverty, conflict, instability, low levels of literacy and gender gaps in education have fuelled child marriage in South Sudan for years, according to campaigners.

Many South Sudanese communities see child marriage as a way to protect girls from pre-marital sex and unwanted pregnancies, or to exchange them for dowry of resources such as cattle.

Child rights groups welcomed Facebook's action in taking down the post - but said the post had been public for several days and ultimately resulted in the illegal marriage of a minor.

This could create a dangerous precedent whereby other families may seek to use social media sites to get greater dowries for their daughters, according to charity Equality Now.

"Facebook has a responsibility on securing and protecting the rights of women and girls," Judy Gitau, a lawyer at Equality Now, told the Thomson Reuters Foundation.

"They need to put sufficient resources into monitoring what is on their platform."

Reuters