SEATTLE (AP) - King County will buy up to 500 articulated buses from New Flyer Industries with most of the buses to be powered by hybrid engines, the county announced Wednesday.

The five-year contract could be worth as much as $400 million, General Motors Corp. said in a news release. GM supplies Winnipeg, Manitoba-based New Flyer with the hybrid system for the buses.

King County has operated a fleet of 214 GM hybrid-powered buses since 2004.

"Its innovative public-private partnerships such as this that put King County in a leadership position to combat global warming," King County Executive Ron Sims said in the news release. "We will put new rapid transit bus service on the streets with the first 122 articulated hybrids ordered. The size of this contract demonstrates our commitment to transit and the resulting benefits to our residents for air quality and traffic congestion."

GM said 720 buses with its two-mode hybrid system have been delivered so far to 56 cities in the United States and Canada.

The hybrid buses have significantly better fuel economy, cut some exhaust emissions by up to 90 percent and have operating sound levels approaching that of passenger cars, GM said.

In the hybrid system, a diesel engine powers a battery, which in turn powers electric motors to run the wheels.