More than 30 cars have been involved in two pile-ups amid snowy conditions in Chicago as monster Storm Stella sweeps in across half of the US.

Forecasters have warned of record low temperatures and dangerous travelling conditions, prompting flight cancellations and school closures in the northeast.

The warnings forced German Chancellor Angela Merkel to postpone a trip to the US for her first meeting with President Donald Trump from Tuesday until Friday.

Almost half the country was caught in an icy blast on Monday after an unusually warm winter.

Some 34 vehicles were involved in two crashes in Chicago, injuring seven people.


Image: People walk along a snow-covered lakefront in downtown Chicago

Police said snowy conditions caused the smashes on two express lanes to the north of the city.

"I don't know if the express lane should have been open," one witness said.

"I slid about 100 yards and thankfully didn't hit anything."

Another described the conditions as "very bad".

"I saw the smash. I stopped, I stopped the car. And I saw from in back they were coming, all the cars that are coming and pile-up and bam bam bam. They hit each other and they hit me too," he added.

Forecasters have warned the next few days will get worse as winter storm Stella which will bring blizzards and heavy snow.

Image: Storm Stella will bring snow to large areas of the US. Pic: NOAA

Up to 18in (45cm) of snow has been predicted covering an area from the central Appalachians to New England.

The storm could bring record-low temperatures late on Monday into Tuesday, as well as "blizzard conditions in places, difficult travel and power outages," The National Weather Service has warned.

A blizzard warning was issued for New York, with snow expected to make travel "extremely dangerous," said forecasters.

Sky's US correspondent Hannah Thomas-Peter, who's in New York, said: "The North East is well used to big winter storms, but this one is forecast to pack a real punch.

"Just a few days ago New York was celebrating what felt like the early arrival of spring - people were wearing shorts and t-shirts in Central Park and daffodils were beginning to emerge from the ground.

"Now this city is preparing for a miserable combination of heavy snow and high winds - a pairing almost guaranteed to knock out power and seriously disrupt travel."

Winter storm warnings are also in place for areas from Washington to southern Maine where winds of up to 60mph are expected.

Image: Snow at a football match in Minneapolis before Storm Stella arrives

The weather service said: "Significant amounts of snow are forecast that will make travel dangerous.

"Only travel in an emergency. If you must travel, keep an extra flashlight, food and water in your vehicle in case of an emergency."

Hundreds of flights were cancelled ahead of the storm, according to the tracking service FlightAware.

Southwest Airlines warned of cancellations, delays and diversions for Sunday through to Wednesday.

Delta Air Lines told travellers they could book alternative flights without fees.