A teenage girl has been sexually assaulted by two men in the stairwell of a car park in Sydney's south-west, police say.

The 16-year-old was outside Liverpool City Library at 9.20pm on Saturday when she was approached by a group of six men.

Police said the girl told them she spoke briefly with the group before they invited her to come with them and meet a mutual friend.

The girl told detectives she was then sexually assaulted by two of the men in the stairwell of the car park. The girl went back to the library where police were called.

The men were seen walking towards the nearby shopping centre.

Detectives on Monday night appealed for anyone with information about the attack to come forward.

"All six males have been described only as being of African appearance and aged about 20 years old," a police statement said.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.