Vatican City: Pope Francis has declared everyone has the right to be baptised, even aliens should they come knocking on the church's door.

Christians cannot "close the door" to all those who seek baptism even if they are "green men, with a long nose and big ears, like children draw," the pope said at his daily mass on Monday, according to Vatican Radio.

Pope Francis during his weekly audience in St Peter's Square earlier this month. Credit:AFP

"If tomorrow, for example, an expedition of Martians arrives and some of them come to us ... and if one of them says: 'Me, I want to be baptised!', what would happen?" Francis said in another display of his lively sense of humour.

The Argentine pontiff known for his down-to-earth style has often used colourful and humorous expressions to make his points on the direction of the Roman Catholic Church.

On baptism Francis has stressed that it should not be refused, especially to children whose parents may be breaking Church rules such as living together as an unmarried couple.

AFP