The fire near 9th Avenue was reported around 1:30 p.m., according to San Francisco fire dispatchers.
Red Cross spokeswoman Cynthia Shaw said it appeared that one unit had been destroyed and a couple others had suffered damage in the fire.
Area resident Tina Wells, who was at Friendly Cuts across the street from the afternoon blaze, said she spotted an ambulance at the scene and emergency personnel placing a cast on someone's leg.
Beau Beausoleil, owner of Great Overland Books, also located across the street from the fire, said he saw smoke billowing out of the building's windows this afternoon.
He described "firemen arriving in large numbers and efficiently doing their job," with the last fire engine leaving the scene at around 5 p.m.
The surrounding street was closed for about an hour and a half this afternoon but has since reopened, Beausoleil said.