The archbishop of Naples claimed today that the dried blood of the city's patron saint began to turn liquid in an ampoule on Saturday while Pope Francis was in the cathedral where the relic is stored.

Faithful believe the "miracle" of St. Gennaro can occur three times a year on certain regular feast days if they pray enough after it is taken out of a vault and put on display.

More scientifically minded sceptics say the "miracle" is due to chemicals present in the ampoule that make the viscosity of the blood change when it is moved, as it was on Saturday when it was removed from the vault for the pope's visit.

"In a sign that St. Gennaro loves the pope ... the blood is already half liquid," the city's archbishop, Cardinal Crescenzo Sepe, told the crowd in the cathedral as he held up the ampoule and waved it.

Francis appeared surprised by the cardinal's announcement. He took back the microphone and reacted to his words with a joke without taking a position on the popular tradition.

"The bishop said the blood is only half liquefied. It appears the saint only loves us half-way. We have to convert ourselves (to good) more so he loves us more," Francis said.

The ritual of the liquefying blood dates back to the 14th century.