Vatican City (AFP) - Pope Francis on Sunday appealed to the international community to redouble efforts to end the "modern slavery" of child labour.

The pontiff was speaking on World Day Against Child Labour, sanctioned by the International Labour Organization.

"We must all relaunch efforts to remove the causes of this modern slavery", he said in his weekly audience in St Peter's Square.

"Millions of children are deprived of some of their fundamental rights and find themselves exposed to grave dangers. There are so many slaves today!"

The ILO estimates there are around 168 million children working around the world, around half of whom are doing hazardous jobs that put their health at risk, such as in mines or in quarries.