NEW YORK (AFP) - Treacherous snow and freezing rain struck the north-eastern United States (US) on Wednesday, disrupting thousands of flights and causing traffic chaos for millions.

More than 600,000 people were without power as high winds and ice knocked down trees and electrical lines.

Most were in Pennsylvania, but New Jersey and New York also experienced significant outages.

A state of emergency was declared in New Jersey and New York, where dwindling reserves of salt - used to melt snow and road ice - was a problem.

By late January, New Jersey had already used 277,000 tonnes of salt, 18,000 more than during the entire previous winter.

New York Governor Andrew Cuomo said the state's highway department had enough salt to deal with the storm and was sending 130 truckloads to help address shortages in New York City and Long Island.

"The State is also working with suppliers to increase salt deliveries to local governments," his office said in a statement.

Connecticut Governor Dannel Malloy on Wednesday ordered all non-essential employees to stay home and a number of schools were also closed.

Heavy snowfall was expected across a swath of the north-eastern US, arriving in New England from the central US. Snow will be followed in several places by freezing rain, forecasters said.

More than 3,000 US flights were cancelled on Wednesday and over 7,000 were delayed, according to FlightAware.com.

Hardest hit were airports in Boston, New York and Chicago.

Little relief from the brutal winter blast was expected anytime soon.

"Snow will come to an end in the Northeast tonight as Arctic air surges down into much of the northern and eastern two-thirds of the country," the national weather service said.

Weather forecasters have warned that yet another winter storm is likely to hit at the weekend.