Fire department spokeswoman Mindy Talmadge said two buildings are involved, and that the fire crept up into the attic area of at least one building. She said firefighters opened up the roof to ventilate the fire.
Talmadge said at about 2:20 p.m. that crews had gained the upper hand on the fire but that it hadn't yet officially been controlled.
No injuries have been reported.
The buildings affected by the fire are at 310 and 314 Walnut St.
Kathleen Plant, who lives next door to 310 Walnut, said she was home sick and taking a nap when she heard noises and saw smoke coming from the adjacent building, then called 911.
Kansas City resident David Buchanan, who is visiting San Francisco and staying in the neighborhood, said he saw a woman come out of one of the burning buildings carrying a dog. She yelled, "Call 911," which he did.
The cause of the fire remains under investigation, and a stretch of Walnut Street near the site of the fire was still blocked off as of 2:30 p.m.