Vatican City (AFP) - Two of the three child shepherds who reported apparitions of the Virgin Mary in Fatima, Portugal a century ago will be declared saints on May 13, Pope Francis announced Thursday.

The canonisation of Jacinta and Francisco Marto will take place during the Argentine pontiff's visit to a Catholic shrine visited by millions of pilgrims every year.

The trip has been timed to mark the 100th anniversary of the first reported appearances of the Virgin who, according to a much-debated tradition, shared three major prophesies with the impoverished, barely-literate children.

Jacinta was seven at the time, her brother Francisco was nine and their cousin, Lucia dos Santos, was 10.

Francisco died in 1919 and Jacinta the following year in the Spanish Influenza epidemic that swept through Europe at the end of World War I.

Believers say they displayed great forbearance in their suffering because the Virgin had told them they would join her in heaven at a young age.

Buried at the Sanctuary of Our Lady of Fatima, they will be the youngest non-martyrs to be canonised, and Thursday's announcement was greeted with joy by the local bishop.

"This will ensure the centenary of the apparitions will take place in all its splendour," said Antonio Marto.

- A second miracle -

After her first visit, Mary is said to have appeared to the children repeatedly over a six-month period, always on the 13th.

Tens of thousands of people gathered for the final one on October 13, 1917, with several witnesses saying they had seen the sun "dance in the sky".

Lucia dos Santos became a nun and in 1941, she reported that she and the siblings had been given three secrets by Mary.

According to interpretations of her accounts, the first was a vision of Hell and the second was a warning that another, more devastating war was looming -- World War II, which by that time had already been under way for two years.

The third secret, which Lucia kept to herself for years, was finally published in 2000. It described a vision of the death of a man robed in white. This came to be seen as a prophecy of the 1981 assassination attempt on Pope John Paul II.

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Dos Santos died in 2005, aged 97. A process that could lead to her also becoming a saint has been opened.

Francis, 80, will be the fourth pope to visit the Fatima shrine, after Paul VI, John Paul II and Benedict XVI. The shrine, located about 130 kilometres (80 miles) north of Lisbon, draws millions of visitors each year.

It was John Paul II who beatified Jacinta and Francisco in 2000 following decades of debate over the events at Fatima and their meaning.

The canonisation follows the approval last month of a second miracle attributed to Francisco and Jacinta's intercession.

According to Vatican Radio it involved the "inexplicable" healing in 2013 of a six-year-old Brazilian child who survived a fall of seven metres, despite suffering serious skull injuries.

The pair's beatification was preceded by the June 1999 approval of an account of wheelchair-bound Maria Emilia Santos regaining the ability to walk on February 20, 1989, the anniversary of Jacinta's death, after praying to her.

- 'Worthy of belief' -

Talk of Fatima and miracles does not sit comfortably with everyone in the Church and Vatican commentaries tend to emphasise the children's heroic virtue rather than the supernatural aspects.

"Little Francisco had a great desire to atone for the offences of sinners by striving to be good and by offering his sacrifices and prayers," John Paul said at the beatification.

"The life of Jacinta ... was motivated by these same sentiments."

The Church officially recognised the Fatima visions as "worthy of belief" in 1930 and authorised the veneration of Our Lady of Fatima.

It has never officially recognised the "dancing sun" incident as a miracle, although Pope Pius II described the apparitions as "miraculous" in a 1954 encyclical.

"I can be a good Catholic and not believe in Fatima because it is not dogma. It is obvious that Our Lady did not appear at Fatima," said Anselmo Borges, a priest who prefers to interpret the story in terms of the three children having an intense "interior religious experience".