CHICAGO (CBS) -- Chicago police were warning about a string of five robberies in 11 days at stores on the South and Southwest sides.

Police said the robberies happened between April 2 and April 12 in the Gage Park, Brighton Park, and Back of the Yards neighborhoods.

In each of the robberies, someone approached the victims at the front counter of a business, showed or implied he had a weapon, and stole money, jewelry, and iPhones from the store before running off.

The robberies happened at the following times and locations:

• At 3:30 a.m. on April 2 in the 5000 block of South Western Avenue.
• At 2:41 a.m. on April 3 in the 4300 block of South Archer Avenue.
• At 11:30 a.m. on April 5 in the 4300 block of South Ashland Avenue.
• At 3:40 p.m. on April 7 in the 5100 block of South Kedzie Avenue.
• At 8 p.m. on April 12 in the 4300 block of South Ashland Avenue.

Police had only a vague description of the robber, who was wearing a black ski mask, black hoodie, traffic vest, and dark sweatpants or blue jeans.

Anyone with information on the robberies is asked to call Area One detectives at 312-747-8384.