Much of the eastern United States has been lashed with intense storms, leaving at least two people dead, hundreds of thousands without power and thousands of flights cancelled or delayed.

Millions of people were under severe weather alerts Monday, including tornado watches, as rain, strong winds and hail swept east along nearly the entire eastern seaboard, from Alabama to New York.

The National Weather Service (NWS) had predicted a "moderate risk" of hazardous storms, with gusts up to 130 kilometres per hour.

"Stay weather aware and make sure you have multiple ways to receive warnings," the NWS in Baltimore and Washington said on social media on Monday.

As much of the severe weather danger subsided into the late evening, some areas faced flood threats as rain continued to fall.

Hail as large as 11.5 cm in diameter was recorded in Virginia, the NWS said.

Fatal extreme weather

In Alabama, a 28-year-old man died after being struck by lightning in an industrial park parking lot, a local ABC station reported.

And in South Carolina, a 15-year-old was killed when he was hit by a falling tree outside his grandparents' house, according to a local CBS station.

By late Monday, more than 700,000 customers had lost power along the East Coast, according to tracking website Poweroutage.us.

More than 1,700 US flights were cancelled on Monday and more than 8,000 were delayed as the severe weather loomed, the website FlightAware said.

In Washington, federal agencies sent employees home early at 1900 GMT in anticipation of the weather.

Scientists say climate change has amplified the intensity and frequency of extreme weather events around the world.