The Vatican has opened an investigation into reported apparitions of the Virgin Mary at the small town of Medjugorje in southern Bosnia which have drawn more than 30 million pilgrims and divided the Catholic Church.

The Congregation for the Doctrine of the Faith, the Church's top doctrinal body, has named an international commission of inquiry headed by Italian Cardinal Camillo Ruini, a confidant of former Pope John Paul, according to the Vatican.

Since six children first reported visions of the Virgin Mary on a hillside near Medjugorje in 1981 - reminiscent of famous apparitions in the French town of Lourdes and Fatima in Portugal - Catholics have debated whether the visions were a modern-day miracle, wishful thinking or an elaborate fraud.

"This commission, composed of cardinals, bishops, theologians and experts, will work in a confidential manner and submit the result of its investigation to the Congregation," the Vatican said in a statement.

Unlike Fatima or Lourdes, the Vatican has not officially recognized the apparitions in the small town, some 100 kolometres southwest of Sarajevo, and claims about it are controversial.

Bishop Ratko Peric of Mostar, the nearest city in Bosnia, warned Catholics last year against uncritical belief in the Medjugorje sightings and issued a series of restrictions on the parish.

In July, Pope Benedict defrocked Rev. Tomislav Vlasic, a Franciscan priest who served as a former "spiritual director" to the six visionaries, after a year-long probe into charges he exaggerated the apparitions and had fathered a child with a nun.

Months later, Vienna Cardinal Christoph Schoenborn, a close ally of Benedict, stirred controversy with a well-publicised visit to the shrine in December that drew a public reproach from Peric.

Cardinal Schoenborn, who issued a letter of apology, had insisted his visit there was private.

Despite clashes between the Franciscan priests running the site and the Vatican, which expelled 10 of them from the order for promoting the site in defiance of its warnings, the village became a pilgrimage destination, giving many visitors a renewed sense of spirituality and locals a steady source of revenue.