Residents in areas of West and Northwest Philadelphia are having problems reaching 911 from landlines, according to the Mayor's Office.

The issue, the cause of which is currently unknown, began Monday afternoon, according to a mayoral spokeswoman. The city's Office of Emergency Management is working with Verizon to identify and resolve the problem, the spokeswoman said.

Cell phone coverage was not affected by the outage, so those unable to reach 911 via a landline are urged to use a mobile phone, if possible.

A Verizon spokesman did not immediately return a call for comment.