CHICAGO (AFP) - After two days of US weather so cold that at least one polar bear at a zoo had to be taken inside, some degree of relief was forecast for Wednesday.

The National Weather Service gave no precise temperatures but said "a much anticipated warm up" was expected over the eastern two-thirds of the United States.

The arctic air mass that has caused this week's bone-chilling cold will begin to moderate, the service said on its web site.

But it will still be long-underwear and scarf weather for many: the weather service predicts temperatures will be 15 to 25 degrees Fahrenheit below average in parts of the upper Midwest.

And air travel that has been a nightmare, with thousands of flights delayed or cancelled outright, still looked tricky.

As of early Wednesday, 551 flights into, out of or within the United States had been cancelled and 206 delayed, according to FlightAware.com, a site that monitors air travel.

The slight warming comes after some truly stunning temperatures - in some cases lower than on the surface of Mars.

It was so cold in Chicago on Tuesday that the polar bear at the Lincoln Park zoo - which had not built up the winter fat stores of her wild cousins - was brought inside to warm up, according to zoo spokeswoman Sharon Dewar quoted by US media.

And in Kentucky, an escaped inmate begged to be let back into prison so he could warm up after spending the night shivering in an abandoned house.

All of Canada and all the US states bar tropical Hawaii recorded temperatures below freezing, even usually sunny and warm Florida and California.

But the most dangerous cold - cold that can cause frostbite in a minute and death in a matter of hours - hit the Midwest, dragged down as the 'polar vortex' brought frigid air from the Arctic.

Schools, businesses and government offices were closed. Water mains and household pipes froze. Airplanes were grounded, trains were halted and roads and sidewalks became ice rinks.