A storm Wednesday dumped more than half a metre of snow on Colorado, bringing much of the state to a halt.

Schools, malls and offices were closed Thursday, the governor declared a state of emergency and 4,700 travellers spent the night at Denver International Airport after flights were cancelled.

The Colorado National Guard was rescuing stranded drivers, who were flooding into shelters.

Mail deliveries were cancelled in the eastern part of the state. Avalanche warnings were issued in the mountains.

A notice on the airport website Thursday morning said it was not likely to open until the evening. More than 1,000 flights were cancelled on Wednesday and Thursday.

The snow is forecast to stop during the day after adding up to five more centimetres, but winds have formed drifts and a major cleanup looms.

The U.S. National Weather Service has a blizzard warning in effect for eastern Colorado and parts of adjacent states - western Nebraska and southern Wyoming. Kansas, to the east of Colorado, is under a storm warning.

Colorado Gov. Bill Owens said declaring an emergency and activating the National Guard would help drivers. The guard also delivered supplies to the Denver airport.

Hundreds of kilometres of highways are closed. Accidents and stalled cars made the mess worse, though there were no traffic deaths, police reported.

About 210 kilometres from Denver in a truckstop in Walsenburg on Interstate 25, the main highway south from Denver, manager Leon Medina described the scene: "Cars are all around the building. Trucks are all over, trucks and cars pulled into ditches."