CHICAGO, Jan. 5 (Xinhua) -- The U.S. Chicago Fire Department announced Friday to put five more ambulances on the street, as a flu outbreak has flooded city's hospital emergency rooms.

Fire Department spokesman Larry Langford said the spike in flu cases in Chicago, the third largest city in the United States, was partly triggered by frigid weather as winter storm is slamming the windy city.

After embracing the coldest day this winter on the New Year's Day, Chicago has stayed in cold and chill since then.

The severe cold has caused a corresponding spike in calls for emergency medical services, prompting response times to skyrocket.

The five additional advanced life support ambulances will give Chicago 80 ambulances capable of delivering the most sophisticated level of care. Currently, there are 75 ambulances on the streets of Chicago.

The number of U.S. states reporting widespread flu activity jumped from 23 to 36, the U.S. Centers for Disease Control and Prevention (CDC) said a week ago.