Millions of girls in Africa are at risk of female genital mutilation because their governments are failing to enforce laws banning the internationally condemned practice, campaigners said on Thursday.

Six countries which are home to 16 million girls - Chad, Liberia, Mali, Sierra Leone, Somalia and Sudan - still do not even criminalise FGM, according to a major report examining laws in the 28 African countries where the tradition is endemic.

"These countries are failing to protect their girls and women," said Ann-Marie Wilson, executive director of campaign group 28 Too Many, which worked with 125 lawyers around the world to compile the study, the largest of its kind.

"FGM is always traumatic and has a life-long impact. A law sends a strong message from the top that this is unacceptable."

World leaders have pledged to end the practice - which involves the partial or total removal of the external genitalia and affects 200 million girls and women - under global development goals agreed in 2015.

But laws in the 22 countries which have banned FGM are mostly inadequate and seldom enforced, campaigners said, adding that prosecutions were rare and penalties too light to act as a deterrent.

"Most laws are not tough enough. There are huge gaps," said Wilson.

Many communities that practice FGM see it as crucial for a woman's social status and a pre-requisite for marriage, but the ritual can cause serious physical and psychological harm.

The report's authors estimate 55 million girls in Africa under the age of 15 have undergone FGM or are at risk. Half live in just three countries - Egypt, Ethiopia and Nigeria - all of which have banned it.

Only two countries, Kenya and Uganda, have robust legislation, according to the report, which was facilitated by TrustLaw, the Thomson Reuters Foundation's legal pro bono service.

It highlights concern that most laws fail to address the growing trend for health professionals to carry out FGM. This is particularly common in Egypt, where there have been several high profile cases of girls dying after being cut at clinics.

Another trend is for parents in countries where FGM is illegal to take girls across national borders to be cut, for instance from Burkina Faso to Mali.

The report urged all countries to criminalise "medicalised FGM" and cross-border FGM.

It also sets out a model FGM law which could be adapted to different country's legal and social environments.

But campaigners said laws could not help end the practice unless they were properly enforced and accompanied by strong initiatives to change attitudes. Girls who opt not to be cut are often abused and ostracised by their communities.

The report recommended countries follow Kenya and Uganda's example by making it a crime to discriminate against or threaten uncut girls.

Some facts about FGM

- FGM dates back over 2,000 years. It is practised across many cultures and religions.

- An estimated 55 million girls in Africa under the age of 15 have undergone FGM or are at risk.

- Somalia has the world's highest FGM prevalence (98 percent of women have been cut), followed by Guinea, Djibouti, Mali and Sierra Leone.

- 22 of the 28 countries in Africa where FGM is endemic have legislation criminalising FGM, although enforcement is generally weak and prosecutions rare.

- Half of all girls who have undergone FGM or are at risk live in three countries - Egypt, Ethiopia and Nigeria - all of which have laws against FGM.

- Chad, Liberia, Mali, Sierra Leone, Somalia and Sudan, which are home to 16 million girls, have no law, meaning FGM is still effectively legal.

- All these countries apart from Sierra Leone have either drafted legislation or indicated they intend to pass a law.

- Fines set out in laws vary from the equivalent of about $5 to $3,600.

- Prison sentences range from a minimum of two months to a maximum of 20 years (Cameroon). But sentences so far have been lenient and frequently suspended.

- There is an increasing trend for FGM to be carried out by health professionals rather than traditional cutters, particularly in Egypt, Guinea, Kenya, Nigeria and Sudan.

- Of these countries, only Guinea and Kenya specifically criminalise medicalised FGM.

- Somalia is unusual in that its constitution prohibits FGM, but it has no law criminalising the practice.

- It recently announced its first prosecution for FGM after a 10-year-old girl died from the procedure.