By Kieran Guilbert

DAKAR (Thomson Reuters Foundation) - Women's rights activists on Friday welcomed Gambian President Yahya Jammeh's decision to ban child marriage, but said jailing parents who marry off their daughters could spark a backlash in a country where a third of girls are wed before they turn 18.

Jammeh announced this week that child marriage in Gambia was banned with immediate effect, punishable by 20 years in prison for both the husband and parents of the girl being married.

People who were aware of child marriage yet chose not to report it could face a sentence of 10 years, Jammeh added.

"We hope that a law will be enacted immediately and that strong action will follow to ensure that every single girl is protected," said Christa Stewart of the London-based women's rights organization Equality Now.

Almost a third of women aged between 20 and 24 in Gambia were married before the age of 18, and nearly a tenth before 15, according to data from the U.N. children's agency (UNICEF).

Yet a law alone will not be enough to stop girls from being married before 18, said campaign group Girls Not Brides (GNB).

"It is essential to empower girls, to protect their rights and provide meaningful alternatives to marriage that are valued by communities, such as education," said Ruth Koshal of GNB.

Gambia last month became the 13th nation in Africa to join the African Union's (AU) campaign to end child marriage, which aims to raise awareness of the risks of the practice.

Early marriage deprives girls of education and opportunities and increases the risk of death or serious childbirth injuries if they have babies before their bodies are ready. Child brides are also at greater risk of domestic and sexual violence.

The government should engage with local communities to try to change attitudes towards child marriage, instead of threatening families with prison sentences, said Isatou Jeng of the Gambian women's rights organization Girls Agenda.

"I don't think locking parents up is the answer ... it could lead to a major backlash and sabotage the ban," she told the Thomson Reuters Foundation by phone from the capital, Banjul.

Female genital mutilation (FGM) was criminalized in December, a month after Jammeh announced a ban on the practice in Gambia, where three in four women have been cut.

While many activists supported the ban, some fear it could drive the deeply entrenched practice underground, unless efforts are stepped up to get religious leaders on board.

(Reporting By Kieran Guilbert, Editing by Ros Russell; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit news.trust.org)