An intensified Hurricane Sandy is roaring upon the US East Coast, threatening major floods and power cuts that could affect 60 million people. Hundreds of thousands of residents have been ordered to evacuate New York City and New Jersey. Public transport has been suspended in cities up the eastern seaboard and thousands of flights grounded. Forecasters fear Sandy will become a super-storm when it collides with cold weather fronts from the west and north. The category one hurricane has already killed 69 people - 52 of them in Haiti - after sweeping through the Caribbean in the past week. Slow-moving storm At the White House, President Barack Obama warned Americans in harm's way to follow emergency instructions. "When they tell you to evacuate, you need to evacuate. Do not delay. Don't pause," said Mr Obama, who has cancelled campaign events eight days before the US elections. "Don't question the instructions that are being given, because this is a serious storm and it could potentially have fatal consequences." The president has signed nine emergency declarations covering New York, New Jersey, Massachusetts, Connecticut, Rhode Island, Delaware, Pennsylvania, Maryland and District of Columbia. The storm threatens an 800-mile (1,290-km) swathe of the US, from the Atlantic Ocean to the Great Lakes in the Mid-West. Up to 3ft (91cm) of snow is expected to fall on the Appalachian mountains in West Virginia, Virginia and Kentucky. At 14:00 EDT (18:00 GMT), Sandy was churning about 170 miles south-east of New York City, the National Hurricane Center (NHC) said. The hurricane strengthened on Monday morning, packing maximum sustained winds of 90mph with higher gusts. Hurricane force winds extended for 175 miles and tropical storm force winds for 485 miles, the NHC added. Authorities said that high tides swelled by a full moon could create storm surges of up to 11ft. The eye of the weather system, which has been dubbed Frankenstorm, is expected to make landfall just south of the New Jersey coast on Monday evening, the NHC said. Forecasters say Sandy could linger over as many as 12 states for 24-36 hours. President Obama cancelled a planned rally in Florida with former President Bill Clinton on Monday and an event for Tuesday in Wisconsin, far from the storm. His Republican challenger, Mitt Romney, held two rallies as planned in the Mid-West, but cancelled events later on Monday and Tuesday. Amtrak has halted passenger train services across the north-east, while almost 9,000 flights were cancelled, according to Flightaware.com. With public transport suspended in New York, Washington DC, Baltimore, Philadelphia and Boston, many workers stayed at home on Monday. Authorities closed the Holland Tunnel under the Hudson River between New York and New Jersey. Maritime operations at shipping terminals in New York City and New Jersey were also suspended.