Female clerics on Thursday issued an unprecedented fatwa against child marriage in Indonesia in a bid to stop young girls becoming brides in the world's most populous Muslim country.

The fatwa - which is influential among Muslims but not legally binding - came at the end of an extraordinary three-day conference of female Islamic clerics: a rare example of women assuming a lead role in religious affairs in this mostly-Muslim country.

"Maternal mortality is very high in Indonesia. We - as female clerics - can play a role on the issue of child marriage," conference organiser Ninik Rahayu told the Thomson Reuters Foundation.

"Female clerics know the issues and obstacles women face, we can take action and not just wait for the government to protect these children," she said by phone from Cirebon in the West Java province, where the congress was held.

Indonesia has one of the worst records for under-age marriage - its high number of child brides puts it among the top 10 countries worldwide - and it is common for girls to marry before they turn 18.

Thursday's fatwa, or religious edict, called underage marriage "harmful" and said its prevention was mandatory.

FALLOUT OF EARLY WEDLOCK

One in six Indonesian girls marry before they turn 18, equal to 340,000 girls a year, according to the United Nations children's agency UNICEF. About 50,000 wed before they turn 15.

A government report last year showed almost a quarter of married women aged 20-24 had entered wedlock when they were under 18.

The Southeast Asian nation has a population of 250 million.

Under Indonesian laws, the minimum age of marriage for girls is 16, and 19 for boys.

In issuing the fatwa, the women clerics cited studies saying many Indonesian child brides could not continue their studies once wed and half their marriages ended in divorce.

They urged the government to raise the minimum marriage age for girls to 18, a demand activists have sought for years.

Early marriage not only makes it more likely that girls will quit school, campaigners say it also increases the risks of exploitation, sexual violence, domestic abuse and death in childbirth.

About 300 participants took part in the congress, which included Indonesian women clerics and women leaders from Afghanistan, Pakistan and Malaysia. Organisers billed the three-day conference as the first meeting of its kind in the world.

The congress also issued two other fatwa against environmental destruction and sexual violence, which the clerics said are against Islamic teaching and fundamental human rights. (Writing by Beh Lih Yi @behlihyi, Editing by Lyndsay Griffiths; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience.