Pope Francis is sending the Vatican's top expert on sex abuse to investigate the case of a Chilean bishop accused of covering up a paedophile priest's crimes.

The decision to investigate Bishop Juan Barros after the pope only recently defended him is because 'some recently received information' has emerged, a statement said.

Now Maltese Archbishop Charles Scicluna, who prosecuted cases involving predator priests for the Vatican's doctrinal body from 2002 to 2012, will travel to Chile to listen to witness statements

The controversy around Bishop Barros dominated his trip to Chile and Peru earlier this month where he hugged and defended the bishop.

Pope Francis told reporters there was no evidence against Bishop Juan Barros Madrid, who he appointed as Bishop of Osorno despite accusations he was complicit in hiding the crimes of Rev Fernando Karadima, Chile's most notorious priest.

'The day someone brings me proof against Bishop Barros, then I will talk,' Francis said before celebrating Mass outside the northern Chilean city of Iquique. 'But there is not one single piece of evidence. It is all slander. Is that clear?'

He later apologised, acknowledging the word 'proof' had hurt many people.

'I know there are many people who are abused and cannot prove anything.'

However, he added that the Vatican had investigated the case and he was 'convinced' of the bishop's innocence.

The latest revelations, seemingly on the basis of new compelling evidence, will raise further questions about Francis' staunch defence of Barros and his status as a bishop in South America.