A woman in central Kenya was jailed for six years for forcing her 13-year-old twin daughters to undergo female genital mutilation (FGM) in a rare conviction in the east African nation, a charity which helped rescue the girls said on Friday.

Florence Muthoni from Tharaka-Nithi county was arrested on Wednesday after a tip-off from the charity Plan International.

She was sentenced by a magistrates court in Chuka on Thursday after admitting to taking her daughters to a circumciser.

A senior aid worker at the charity said Muthoni told the court that she wanted her daughters to undergo FGM to avoid a curse from her deceased grandfather who had instructed all girls in the family undergo the procedure.

"A community member alerted us when they had heard the mother was organising the girls to undergo the cut, so we informed the local authorities," Mercy Chege, a director at Plan International, told the Thomson Reuters Foundation.

"Unfortunately, we were not able to prevent the circumcision as by the time the police conducted the raid and rescued the girls, they had already been cut."

The twin girls are receiving medical treatment and counselling while police are still investigating as the mother had refused to name the circumciser, said Chege.

According to the United Nations, one in five women and girls aged between 15 and 49 in Kenya have undergone FGM, which usually involves the partial or total removal of the genitalia.

In some cases, girls can bleed to death or die from infections.

FGM can also cause lifelong conditions such as fistula as well as fatal childbirth complications.

Kenya outlawed the practice in 2011, but it continues as communities believe it is necessary for social acceptance and increasing girls' marriage prospects.

While some arrests have been made and cases brought to court, campaigners say implementation of the law remains a challenge, largely due to a lack of resources and capacity of law enforcement agencies and difficulties reaching remote areas.

U.N. data shows 75 cases of FGM were brought before Kenyan courts in 2016 but only 10 cases resulted in a conviction.

Campaigners said this week's conviction proved that public awareness campaigns run by charities were essential to curbing FGM as they could lead to community members reporting the crime.

"It is very important that FGM laws are properly implemented as this sends a message out that FGM will not be tolerated," said Ann-Marie Wilson, executive director of 28 Too Many.

The U.N. estimates 200 million girls and women worldwide have undergone FGM. It is practised in about 27 African nations, parts of Asia and the Middle East - and is usually carried out by traditional cutters, often with unsterilised knives.