Media playback is unsupported on your device Media caption New York Mayor Bill De Blasio: "This storm... is going to pack a real punch"

An emergency has been declared in several US states as a storm bringing hurricane-force winds and 90cm (36ins) of snow barrels down on the north-east.

Boston, New York and Philadelphia began shutting down Monday evening as the flurries began, with the worst ahead.

Non-emergency vehicles have been banned on New York City's 6,000 miles of roads after 23:00 local time.

"Recognise this as an emergency, this is not business as usual," said Mayor Bill de Blasio.

New York, New Jersey, Connecticut, Rhode Island and Massachusetts have declared states of emergency and 50 million people are expected to wake up to a foot of snow on Tuesday.

In other developments:

6,500 flights in and out of airports along the East Coast cancelled

schools and businesses stopped early on Monday

New York's subway and bus services stop at 23:00 local time

wind gusts of 75 mph or more are forecast for coastal areas of Massachusetts

Boston's Logan Airport said there would be no flights after 19:00 local time

Glenn Field of the National Weather Service in Taunton, Massachusetts, told the BBC on Monday afternoon that the storm will be worse than previously thought.

Hurricane-force winds of up to 80 mph (129km/h) will batter Cape Cod, Nantucket and Martha's Vineyard, he said.

Image copyright AFP Image caption A tugboat on the East River with an obscured skyline behind

Image copyright EPA Image caption Workers lay salt on the city's walkways

Image copyright European Photopress Agency Image caption Stocks of water are running low in some New York shops

Mr Field said the heaviest snowfall will come in the early hours of Tuesday, with 15 inches expected between 0100 and 0500 local time, and 30 inches in total in parts of Massachusetts.

During a Monday afternoon press conference, New York City Mayor Bill de Blasio urged residents to stay out of the way of the 2,300 snowploughs clearing city streets.

Media playback is unsupported on your device Media caption BBC Weather explains the causes behind the coming blizzard

"You can't underestimate this storm," the city's mayor said. "What you are going to see in a few hours in something that is going to hit very hard and very fast."

Similar bans for non-emergency vehicles will be in effect later this evening for the states of Connecticut and Massachusetts and would be likely in the rest of New York state.

Image copyright AP Image caption Snowploughs are out in force in Pennsylvania

Image copyright European photopress agency Image caption Millions of people are in the path of the storm

Image copyright Paul Zepeda Image caption Queues were out the door at Trader Joe's in Union Square

Image copyright Reuters Image caption There are empty shelves in many shops along the East Coast

At least 28 million people will face blizzard conditions over the next day and an estimated 50 million people could see more than a foot of snow in the storm.

"In addition to heavy snow, with blizzard warnings, there's a big threat of high, damaging winds, and that will be increasing Monday into Tuesday," said Bob Oravec, a National Weather Service forecaster. "A lot of blowing, drifting and such."

BBC News website readers prepare

Christie Craighead and her daughter are preparing to evacuate their home in Concord, New Hampshire, to stay with friends who have an electricity generator.

"I have never seen the shops so busy - not even at Christmas or Thanksgiving. We got the last gallon of water, now they just have bottles. The shelves are almost empty, there are no staples like bread left."

Anne McCarthy is originally from Leigh in Greater Manchester, but now lives in Boston in the United States - she has been preparing for the blizzard ahead.

"Everyone is preparing for the electrical outages we've been promised. We have lots of food that doesn't need cooking and water."