CHICAGO, Feb. 9 (Xinhua) -- All public schools in the third biggest U.S. city of Chicago were closed on Friday as severe winter storm hit the lower Great Lakes region.

By Friday morning, as much as six inches (15 cm) of snow has fallen in Chicago, causing dozens of crashes on express ways and massive cancellations of flights to and from the city's two main airports.

As of 1650 GMT, 750 flights at O'Hare International Airport and 314 flights at Midway International Airport were canceled due to the storm, according to flightstats.com.

The U.S. National Weather Service said additional snow accumulations of two to four inches (5-10 cm) are expected in Chicago area till Friday evening.

Mayor Rahm Emanuel told a press conference on Friday morning that all city departments are working hard to deal with the severe winter weather, and that safety is always the priority.

Six warming centers are in operation, where Chicagoans can go to find safe refuge from extreme weather, said officials.

Additional facilities are opened such as senior centers and public libraries.

The winter storm is hitting the Detroit region as well, forcing the closure of hundreds of local schools. Some 250 flights at the Detroit Metropolitan Wayne County Airport were also canceled.

The snow will continue to be steady all day in Detroit area in the midwestern U.S. state of Michigan, said forecasters.

This round of severe winter weather is continuing to affect a large region around lower Lake of Michigan, including northern Illinois, southern Michigan and northern Indiana, till Monday morning.