VATICAN CITY, Oct 10 (Reuters) - Pope Francis on Sunday met a Colombian nun who was freed after being kidnapped by Islamist militants in Mali four years ago.

The Vatican said in a statement that Francis met Sister Gloria Cecilia Narvaez in St. Peter's Basilica just before he celebrated Mass there to open a two-year worldwide consultation on the Catholic Church's future.

The Mali presidency announced her release on Saturday but the timing and other details were not clear.

Kidnapping has been a lucrative source of cash for Islamist groups in West Africa's Sahel region, where they are waging an expanding insurgency against national armies, French forces and U.N. peacekeepers.

Reporting by Philip Pullella Editing by Frances Kerry