SAN FRANCISCO -- San Francisco police Wednesday arrested a 14-year-old boy on assault charges for his alleged involvement in a pit bull attack Tuesday afternoon that injured two teenage girls in the Bayview district.

According to police, two sisters, 13 and 14, were walking near Jennings Street and Revere Avenue around 4:30 p.m. when an unleashed pit bull accompanied by a 14-year-old boy approached them on the sidewalk.

The dog attacked both girls, biting one on the arm and another on the stomach, investigators reported.

An unidentified man apparently came to the girls' aid, striking the dog with a wrench-like tool.

According to police, the teenage boy may have provoked the brown and gray female pit bull to attack the girls.

The dog and the boy fled the scene, but were later apprehended by police. The teenager was booked into the Youth Guidance Center on two counts of assault with a deadly weapon.

San Francisco Animal Care and Control impounded the animal.

"We do have the dog in our custody. We are holding the dog for the required observation period," which is 10 days, spokeswoman Deb Campbell said.

A vicious dog hearing should be scheduled "soon" at San Francisco City Hall, Campbell said, and if the unidentified owners relinquish the dog, "we would then determine what to do," she said.