At least six police officers have been shot in Philadelphia, officials have said.
Police commissioner Richard Ross said a “pretty horrible situation [was] unfolding” and “is not resolved” as a man continues to shoot at officers in the Nicetown area of the Pennsylvania city.
Mr Ross said police were serving a narcotics warrant and had entered the premises when “gunfire erupted and the shooter fired multiple rounds”.
Officers returned fire and attempted to escape to safety by diving through windows and doors.
He confirmed that six officers have been injured but are in a stable condition, with one policeman wounded when a bullet grazed his head.
The incident began at around 4.30pm local time (9.30pm GMT).
Hours later, police are still locked in attempts to negotiate with the shooter, who continues to fire out of a window. Officers fear there may be a potential hostage involved and two police officers are still in the building with the armed man.
“We are trying to talk to this male and that he can end this peacefully,” Mr Ross told reporters, adding that the man has picked up the phone “several times” when police have rung but not spoken.
Asked about two officers inside the house, Mr Ross said: “We believe they’re OK, and I’m not going to say much more about that right now, out of concern for their safety.”
People are being urged to avoid the area.
Footage shows a significant police presence, with dozens of police cars and armed officers.
Part of Temple University had been on lockdown for three hours, during which officials from the institute urged people to avoid the area.
The account posted: “Shots fired reported at 1500 Block Erie Av at Health Sciences Center Campus. Use caution. Avoid the area. Police are responding.”
It added: “Lockdown is in effect for Health Sciences Center Campus. Seek shelter. Secure doors. Be silent. Be still. Police are responding.”
The Bureau of Alcohol, Tobacco, Firearms and Explosives said a number of its agents are on the scene.