A woman was sexually assaulted at gunpoint Tuesday evening in the Burnside neighborhood on the South Side.

The woman, 29, was walking under a railroad viaduct at 6:30 p.m. in the 9400 block of South Woodlawn Avenue when a man approached and asked for money, Chicago police said.

When she said she did not have any, the man pulled out a silver-colored revolver and forced her to the Norfolk Southern tracks on top of the viaduct, where he sexually assaulted and threatened to kill her, police said.

The man, in his early 20s, wore a black mask and fled southeast from the tracks, police said.

Anyone with information was asked to call detectives at (312) 747-8271.