Cambodia has pardoned and sent home 13 Philippine women who were jailed over a scheme to become surrogate mothers, an outlawed practice, the Philippine foreign ministry said on Sunday.

They were among 24 foreign women detained by Cambodian police in September and convicted and sentenced to four years in prison on December 2 for attempted cross-border human trafficking.

The women were sent back home to Manila early Sunday.

"All 13 departed Phnom Penh and arrived safely in Manila following the grant of royal pardon by His Majesty Preah Bat Samdech Preah Boromneath Norodom Sihamoni," the ministry said in a statement.

The Cambodian court ruling had said it had strong evidence showing that the 13 had "the intention... to have babies to sell to a third person in exchange for money, which is an act of human trafficking".

The court did not give details on what would happen to the babies of the 13 when they were born.

In 2016, Cambodia issued a snap ban on commercial surrogacy after neighbouring Thailand pulled the plug on the trade the previous year -- putting an abrupt end to a thriving industry for hopeful parents, many from Australia and the United States.

But demand for commercial surrogacy remains high after China eased its one-child policy and agencies in Cambodia continue to offer the service.

Sources in the kingdom have previously told AFP that couples — mostly from China — are willing to pay between $40,000 to $100,000 to surrogacy agents to find a Cambodian woman who can carry their child.