Sandy regained hurricane strength on Saturday after briefly weakening to tropical storm status, the National Hurricane Center said.
Air Force hurricane hunter planes registered sustained winds of 75 miles per hour (120 kilometers) as the storm churned northward off the US East coast, the US weather agency said.
Dubbed a potential "Frankenstorm," Sandy was expected to make landfall early next week during the frenzied final week of campaigning before the US elections on November 6.
At 1200 GMT, it was about 335 miles (540 kilometers) southeast of Charleston, South Carolina moving in a northeasterly direction at a speed of 10 miles (17 kilometers) per hour.
"Data from NOAA and US Air Force reserve hurricane hunter aircraft indicate that Sandy has maximum sustained winds near 75 mph (120 km/h) with higher gusts," the center said.
Hours earlier, the center said the storm had weakened to tropical storm strength but was expected "to remain a large storm with widespread impacts into early next week."