Private ambulance companies will be at the disposal of the municipal 911 system in case of major disasters or "mass casualty incidents," under a program announced by Mayor Nutter on Wednesday.

Dispatchers will be able to summon the fleet of 300 private ambulances currently working in the city to aid the fleet of 35 to 50 city medic units that operate at any given time. City officials described it as "a new milestone" in public-private EMS cooperation.

With the aid of a $400 state-funded rebate to cover half the cost of each new radio, 80 private vehicles are now capable of communicating with the city's system and being on call as "surge ambulances," as Nutter called them. That includes volunteer outfits such as the Burholme Emergency Medical Service.

Everett Gillison, the city's deputy mayor for public safety, described the conditions that would call for the ambulances as "an unusually large number of casualties" or an incident that required the evacuation of a hospital, for instance.

"The city has conducted multiple tests, and the system works," Fire Commissioner Lloyd M. Ayers said.

Amid the ongoing controversy over rotating closures of fire companies to save money, union leaders questioned whether the move was a step toward privatization. The city handled 221,000 emergency medical service calls last year and its emergency medical service is stretched to the limit.

"We'd like a real clear definition of what a 'mass casualty incident' is," said Jerry Kots, a trustee of Local 22 of the International Association of Fire Fighters.

George A. Butts Jr., Philadelphia's regional emergency medical services chief and architect of the system, said the plan was "scalable" and could be used at varying levels of demand.

But Gillison said, "We're not using this to privatize EMS services." He said the city's agreement with the Federal and Pennsylvania Emergency Management Agencies is specifically for large, surge-type events.