Child marriage became illegal in the Philippines on Thursday as a law banning the practice took effect in a country where one in six girls enters wedlock before the age of 18.

The impoverished Southeast Asian country has the 12th highest number of child marriages in the world, according to Britain-based rights group Plan International, with long-held cultural practices and gender inequalities hindering change.

But the new law, signed by President Rodrigo Duterte and released to the public on Thursday, lays out prison terms of up to 12 years for marrying or cohabiting with anyone under 18.

People arranging or solemnising underage unions also face the same penalty.

"The state... views child marriage as a practice constituting child abuse because it debases, degrades, and demeans the intrinsic worth and dignity of children," the law states.

The government says the law is consistent with international conventions on the rights of women and children.

However, some portions of the legislation have been suspended for one year to allow for a transition period for Muslims and indigenous communities in which child marriage is relatively common.

A report last year by the United Nations Children's Fund said more than half a billion girls and women worldwide were married in childhood, with the highest rates found in sub-Saharan Africa and South Asia.

But recent data indicates the practice is generally in decline on average across the globe.