Chicago police have issued a warning about a string of robberies in the city's Loop and South Loop neighborhoods in which armed robbers demand that victims reset their cell phones to factory settings before stealing them.

In each of the three reported incidents, police said two men confronted the victims on the sidewalk, showed a handgun, and demanded cell phones, wallets, and purses. They then forced the victims to unlock their phones and reset them, as well as demanded PIN numbers of bank cards, according to police.

The three incidents occurred:

on Aug. 29 at 1 a.m. in the 900 block of South Michigan Avenue

on Aug. 31 at 12:30 a.m. in the 1200 block of South Lake Shore Drive

on Sept. 1 at 1:15 a.m. in the 500 block of South Columbus Drive

In one robbery, the offenders were described as black males between the ages of 15 and 17, both standing at around 5'7" and weighing approximately 150-160 pounds. One had a black baseball hat with yellow writing, police said, and the other was wearing a black hooded sweatshirt and blue jeans.

In another incident, both men were described as black males between the ages of 17 and 22, according to police. The first stood approximately 5'6" tall, weighing around 160 pounds, and was wearing a black baseball hat with a Batman logo, authorities said. The second was about 6' tall, weighing approximately 150 pounds, and was wearing black pants, according to police.

Anyone with information on any of these incidents is asked to contact detectives at 312-747-8384.