Rio de Janeiro's state police force has banned officers' use of smartphones on the job, saying instant messaging services and social media on the beat threatens security.

In an internal memo circulated last week, the Brazilian force said it would be monitoring for smartphone and tablet use, which would be considered a "serious offense."

However, the memo, dated Oct. 9 and sent to the Associated Press late Wednesday, did not spell out the possible consequences for offenders.

Brazil is one of the world's top users of social media, and the instant messaging application WhatsApp has become the preferred method of communication for many here.

It's not unusual to see police officers glued to their phones while on patrol or while manning checkpoints, their attention riveted to the screen as snarled rivers of cars trickle by.

The memo said that use of instant messaging apps and social media harms "security, productivity and the quality of (officers') services to population." It told officers to use radios instead.