An Arctic blast sweeping Canada and parts of the United States this week closed schools, grounded jets, killed at least four people, and even shook the ground.

"What the hell was that?" screamed a startled woman at a loud crack produced by a "frost quake" in Ottawa Wednesday night.

Frost quakes, or cryoseisms, occur when frozen water-saturated ground cracks in extreme cold.

The University of Toronto Climate Lab tracked dozens of such events, mostly in Ottawa and Montreal regions, but also in a few US states.

Temperatures plunged as low as minus 40 degrees Celsius/Farenheit with a windchill on Thursday.

The cold is considered dangerous at this level, freezing exposed skin within minutes.

Officials in both countries urged people to stay indoors and not to travel unless absolutely necessary. Pet owners were also told not to let dogs or cats out.

People who must venture out were told to dress in layers covering all skin, to stay dry, to seek shelter and not drink alcohol which gives people a false sense of warmth.

Temperatures dropped at the beginning of the week and the cold is forecast to linger until at least Saturday.

In Ottawa, most bureaucrats stayed home after temperatures plunged overnight following several days of snow and freezing rain.

Ice on streets sent cars sliding into ditches as road crews worked around the clock to clear snow and salt roadways -- with little success.

Others were unable to get their cars started as engines were frozen solid.

Hundreds of flights were delayed or cancelled at major airports throughout the week.

Homeless shelters in most Canadian cities were packed full and hundreds of schools were closed across the continent.

In places where schools remained open, bus drivers in some cases picked up students directly from their homes.

Biting cold temperatures, bringing snow and freezing rain, were recorded as far south as Louisiana, Mississippi and Florida.

As much as a foot (30 cm) of snow fell or was forecast in some US southern and western plains states.

New York Mayor Bill de Blasio warned of the risk of carbon monoxide poisoning from automotive exhaust, home heating systems and obstructed chimneys as well as poorly vented generators, kerosene heaters or gas grills.

In Detroit, firefighters' emergency radios were reportedly freezing to their coats.

In New Jersey, police were empowered to move homeless people off the streets and into shelters.

Extreme winter weather was also blamed for an 18-vehicle pile up on a Pennsylvania highway that left two people dead and dozens injured.

In Toronto, street cars were stopped in their tracks after brake lines and doors froze, leaving as many as 250,000 riders stamping their numb feet on curbsides.

"The aging streetcar fleet and related equipment -- over 30 years (old) in many cases -- do not respond well to extreme cold," the Toronto Transit Commission said on its website.

The cold also sparked a political row between Toronto's mayor, John Tory, and public health officials, over when to issue an extreme cold alert.

When temperatures drop below minus 15 Celsius (5 degrees Farenheit) the city typically opens emergency warming centers and reaches out to homeless people, providing additional shelter beds, transportation and other services to try to get them out of the cold.

Public health officials were accused of having waited too long to issue an alert after two men believed to be homeless froze to death -- one in a bus shelter, the other in an abandoned truck -- this week.

"We're very sorry that these men died," Howard Shapiro, Toronto's associate medical officer of health, told the daily Globe and Mail.

A third man discovered unconscious outside overnight Wednesday was rushed to hospital in critical condition.