Tropical Storm Isaias triggered rare tornadoes and knocked out power as it raced up the US East Coast, killing at least one man in New York City and two others in North Carolina where a twister obliterated a mobile home park.

The fast-moving storm overturned cars, flooded streets and toppled power lines, leaving at least 1.4 million customers without electricity from North Carolina to New York.

In a country already reeling from the coronavirus pandemic, New York state officials shut down testing centers for Covid-19 as a precaution.

Stunned northeasterners watched as tornadoes touched down and recoiled as officials announced warnings for a phenomenon more common in the South or Midwest of the US.

New York City, much of New Jersey, all of Massachusetts and other parts of New England went under a tornado watch.

Social media images showed tornadoes in Cape May, Marmora and Long Beach Island along New Jersey's southern shore, and tornado damage in Dover, Delaware.

In Bear, Delaware, one resident said she was terrified to hear a tornado destroy her backyard.

"The damage is crazy. No power, almost all my trees are split or uprooted, water is coming in all windows," she said on Facebook.

The centre of the storm skirted west of New York City and was about 160km to the northwest as of 3pm Eastern time (8pm GMT+1), the National Hurricane Center said.

One man died in the borough of Queens when a tree fell on his car and crushed it, officials said.

The storm made landfall as a hurricane in North Carolina late yesterday, killing two people at a mobile home park that was reduced to a field of debris.

"There's nothing there," Bertie County Sheriff John Holley told WNCT television.

"Vehicles are turned over. Vehicles are piled on top of each other. It's just very sad."