"Police are regularly called regarding people carrying what appears to be a firearm, which later turns out to be a toy or imitation.
"Such calls will always prompt a full and immediate response from police, include the deployment of armed staff," he said.
Anyone found carrying the item could face a $4000 fine, imprisonment, or both, police warned.
The NYPD 112 Precinct posted a picture of the offending cases on Twitter with the message: "I would NOT suggest purchasing this cell phone case, which was designed to look like a firearm. #BeSmart #BeSafe."
The accompanying image, taken by the Queens North NYPD School Safety Division, shows what looks like a handgun tucked into the back pocket of a woman's shorts, but is actually a mobile phone.
Gun crime has been in the headlines in America, following the massacre of nine people at a church in Charleston last month, which reignited the debate over gun laws.
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