As temperatures plummet, the Vatican has stepped up the help it offers to the homeless sleeping on the streets in Rome.

Vehicles are being left outside the walls of the Vatican with their doors unlocked so that those sleeping rough can take turns sheltering in the warm, according to the Catholic Herald.

Thermal sleeping bags have also been distributed, and Vatican-run shelters are now open 24 hours a day instead of only overnight.

The Pope over the weekend urged Catholics to pray for "people who live on the streets, struck by the cold and, many times, by indifference".

"Unfortunately, some have not survived. Let us pray for them and ask the Lord to warm our hearts so that we can help them," he said.

Pope Francis has made the plight of the needy a central cause of his papacy.

He has had public showers for the homeless built just off St Peter's Square, and last Friday gave 300 homeless people and refugees a sandwich and a drink to thank them for helping to hand out pamphlets at an Epiphany feast day service.

During a special Mass for the homeless in Rome last November, the Pope asked 4,000 homeless or formerly homeless people to forgive Christians who had turned away from helping them and "looked the other way".