The northeastern United States dug out Monday from an unseasonable snowstorm with officials warning families against Halloween trick-or-treating in extreme conditions that left 12 people dead.

Some two million homes were without power across the region, where officials confirmed five storm-related deaths in Pennsylvania, three in Massachusetts, and two each in Connecticut and New Jersey.

Amid the mass power outages, some officials were warning families against traditional Halloween trick-or-treating.

Authorities in the Massachusetts town of Worcester said wandering outside would "put families and our youth in harm's way as they negotiate piles of snow and downed limbs."

Air and rail traffic was returning to normal after a weekend of heavy delays at New York and New Jersey airports and on train routes up the densely-populated east coast from Washington to Boston.

Planes idled on runways, including a travel nightmare for over 100 passengers stranded on a grounded Jet Blue flight in Hartford, Connecticut for seven hours.

In New Jersey, around 400,000 people were still without power.

Governor Chris Christie said state officials estimated that 95 percent of homes would have lights back on by the end of the week and blamed the widespread outages on fallen trees that had cut through power lines.

Further north in Connecticut, officials said over 800,000 households were still without power and Governor Dannel Molloy warned it would be days before power could be restored.

Snow had transformed the region on Sunday into a Halloween winter wonderland.

New York's Central Park saw an unprecedented 2.9 inches of the white stuff. Experts said there hadn't been an inch of snow there on an October day since records began in 1869.