The UN's security service said it was aware of an increase in incidents of theft committed by the fake cops.
“In the majority of cases, two or three men in civilian attire (wearing suits) approach their victim and briefly flash a document which resembles a police ID,” André Bouchard, chief of the UN Office at Geneva's security service said in an email sent to staff on Tuesday.
“They ask the victim to present his or her identity documents, as well as their wallet or handbag, which they ‘inspect', in order to discreetly rob the victim of part or all of the cash found therein,” Bouchard said.
He said the victims usually do not realize they have been robbed until after the impostors have fled.
The impostors are “known to use ingenious methods of distraction,” the email said.
“Sometimes, an accomplice may approach the victim, asking him or her to take a photo, or to confirm the name of a street,” Bouchard said.
“The fake police then arrive and accuse the accomplice of some sort of infraction and proceed with an identification check, as well as that of the victim.”
According to the warning, the criminals posing as Geneva police are not known to resort to violence “and often operate in tourist areas, and near the international organizations and hotels".
The UN security service advises staff confronted with such situations to take a few precautions, including verifying the ID of anyone claiming to be a police officer.
A genuine ID includes a photo of the officer, first and last name, and the symbol of the “République et Canton de Genève” in addition to the word “Police”.
A distinctive yellow and red line also crosses the entire card diagonally, the security service said.
All Geneva police officers must also be wearing a badge.
UN staff were advised in cases of doubt not to hand over their personal belongings and to call 117, the Swiss number for police.
Geneva cantonal police earlier issued a warning about a similar scam involving crooks working in teams of two or more targeting female motorists in particular.
An individual attracts the attention of the driver by mentioning that one of the vehicle's tires is deflated or flat, or that a cat is under the car, police said.
When the driver gets out of the car to inspect the problem an accomplice (or accomplices) open the passenger door to steal a handbag or other valuables that may be inside.
To avoid falling victim to such schemes, police advise women to never leaving handbags or valuables on the passenger seat of the car, recommending that they be placed under a seat or behind the driver's seat.
Another piece of advice is to always keep the passenger door locked and to lock all doors when exiting the vehicle, or to hold on to your handbag, keeping it in front of you when you get out.