Flash flooding caused by the remnants of Hurricane Ida killed at least 44 people in four northeastern US states overnight into Thursday, including several who perished in basements during the "historic" weather event officials blamed on climate change.

Record rainfall, which prompted an unprecedented flash flood emergency warning for New York City, turned streets into rivers and shut down subway services as water cascaded down platforms onto tracks.

"I'm 50 years old and I've never seen that much rain ever," said Metodija Mihajlov whose basement of his Manhattan restaurant was flooded with three inches of water.

"It was like living in the jungle, like tropical rain. Unbelievable. Everything is so strange this year," he told AFP.

Hundreds of flights were canceled at LaGuardia and JFK airports, as well as at Newark, where video showed a terminal inundated by rainwater.

President Joe Biden declared emergencies in the states of New York and New Jersey on Thursday, ordering federal disaster management agencies to coordinate relief efforts and provide emergency support.

The Federal Emergency Management Agency was mandated "to identify, mobilize, and provide at its discretion, equipment and resources necessary to alleviate the impacts of the emergency," the White House said in a statement.

Ahead of a visit to the southern state of Louisiana, where Ida earlier destroyed buildings and left more than a million homes without power, Biden said "we're all in this together. The nation is ready to help".

'Historic weather event'

Flooding closed major roads across New Jersey and New York boroughs including Manhattan, The Bronx and Queens, submerging cars and forcing the fire department to rescue hundreds of people.

At least 23 people died in New Jersey, Governor Phil Murphy told reporters.

"The majority of these deaths were individuals who got caught in their vehicles," he said.

A state trooper died in the neighboring state of Connecticut.

Thirteen died in New York City, including 11 who could not escape their basements, police said. The victims ranged from the ages of two to 86.

"Among the people MOST at risk during flash floods here are those living in off-the-books basement dwellings that don't meet the safety codes necessary to save lives," lawmaker Alexandria Ocasio-Cortez tweeted.

"These are working class, immigrant, and low-income people & families," she added.

Three also died in the New York suburb of Westchester, while another four died in Montgomery County outside Philadelphia in Pennsylvania, a local official confirmed.

Ida blazed a trail of destruction north after slamming into Louisiana over the weekend, bringing severe flooding and tornadoes.

"We're enduring an historic weather event tonight with record-breaking rain across the city, brutal flooding and dangerous conditions on our roads," New York Mayor Bill de Blasio said late Wednesday.

The National Weather Service issued its first-ever emergency flash flood warning for New York City, urging residents to move to higher ground.

"You do not know how deep the water is and it is too dangerous," the New York branch of the National Weather Service (NWS) said in a tweet.

The NWS recorded 3.15 inches (80 millimeters) of rain in Central Park in just an hour -- beating a record set just last month during Storm Henri.

The US Open was also halted as howling wind and rain blew under the corners of the Louis Armstrong Stadium roof.

Lingering tornado threat

New Yorkers woke to clear blue skies Thursday as the city edged back to life, but signs of the previous night's carnage weren't far away: residents moved fallen tree branches from roads as subway services slowly resumed.

By Thursday evening, around 38,000 homes in Pennsylvania, 24,000 in New Jersey and 12,000 in New York were without power, according to the website poweroutage.us, a significant decrease from earlier in the day.

It is rare for such storms to strike America's northeastern seaboard and comes as the surface layer of oceans warms due to climate change.

The warming is causing cyclones to become more powerful and carry more water, posing an increasing threat to the world's coastal communities, scientists say.

"Global warming is upon us and it's going to get worse and worse and worse unless we do something about it," said Democratic senator Chuck Schumer.

In Annapolis, 30 miles (50 kilometers) from Washington, a tornado ripped up trees and toppled electricity poles.

The NWS warned the threat of tornadoes would linger, with tornado watches in effect for parts of southern Connecticut, northern New Jersey, and southern New York as Ida tracked north through New England.

A tornado struck the popular tourist destination Cape Cod, Massachusetts, on Thursday evening.