'A hero is just a sandwich," veteran bus driver Steve St. Bernard of the New York Metropolitan Transportation Authority said after saving a seven-year-old girl from her fall from a building air-conditioning unit. (Metropolitan Transportation Authority)

A New York City bus driver was being hailed as a hero for saving a seven-year-old girl who fell three storeys from an air conditioning unit outside a Brooklyn building.

"I just prayed that I'd catch her," Stephen St. Bernard recalled after rescuing the child on Monday.

"'Please let me catch her, please let me catch her.' That's all I could say," he added.

St. Bernard said he was walking home from work when he observed a commotion outside the Coney Island housing complex. He saw the girl standing on the air conditioning unit, seemingly unafraid and moving about.

It wasn't clear how she got there, but witnesses told the New York Daily News that she crawled out by pulling aside the unit's accordion-like plastic partitions that keep the air conditioner secured in the window.

An amateur video shows St. Bernard yelling up to the girl, telling her to go back inside, when she suddenly falls and he catches her in his arms.

The Metropolitan Transportation Authority says in a website tribute to St. Bernard, a 10-year veteran bus driver and father of four, that he suffered a torn tendon in his shoulder while saving the girl, who was taken to hospital with minor injuries.

"I picked her up and carried her. ... She kept looking around. She never closed her eyes. She never lost consciousness," he said.

Asked if he considered himself a hero, St. Bernard replied, "No. A hero is a sandwich. I just did what I would do for one of my own kids."

Neighbours said the girl was a special-needs child.

Police said no charges were filed against the parents.