The Bronx borough of New York City after it was hit by Hurricane Ida. AFP

New Yorkers were inundated by flash flooding and heavy rain on Thursday as Storm Ida wrought further havoc on the northeast United States, as mayor Bill de Blasio declared a state of emergency due to what he called a "historic weather event.”

At least eight deaths were reported from the flooding, seven in New York City and one in Passaic, New Jersey.

Ida slammed into the southern state of Louisiana over the weekend, bringing severe flooding and tornadoes as it blazed a trail of destruction in the north.

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New York governor Kathy Hochul declared a state of emergency as the remnants of the storm caused massive flooding in the US' financial and cultural capital, leaving the boroughs of Brooklyn and Queens inundated by the deluge.

"Take shelter NOW. Flying debris will be dangerous to those caught without shelter. Move to a lower floor and stay away from windows," tweeted New York City's emergency notification body.

A person struggles with the rainfall in the wake of Hurricane Ida in the Bronx borough of New York City. AFP

Earlier CNN reported that nearly all New York City subway lines were suspended late on Wednesday as the remnants of Tropical Storm Ida brought drenching rain and the threat of flash floods and tornadoes to parts of the northern mid-Atlantic.

Extreme weather also forced the closure of airports in the state and neighbouring New Jersey.

Hundreds of flights have been cancelled at nearby Newark, LaGuardia and JFK airports and flooding has closed major roads across boroughs of the metropolis, including Manhattan, The Bronx and Queens.

The city earlier issued a rare flash flood emergency warning, urging residents to move to higher ground.

A delivery worker wends his way through traffic amid the rainfall on Intervale Avenue in New York City. AFP

"Significant and life-threatening flash flooding is likely from the Mid-Atlantic into southern New England," the National Weather Service said in a bulletin, adding three to eight inches of rain could drench the region through Thursday.

In Annapolis, 30 miles (50 kilometres) from the US capital, a tornado ripped up trees and toppled electricity poles.

Flooding elsewhere in Maryland killed a 19-year-old man and left another person missing after a building was inundated on Wednesday, bringing the death toll from Ida to seven.

New York City also experienced flooding, with social media images showing water gushing over subway platforms and trains.

Subway service was "extremely limited" due to the flooding, the Metropolitan Transit Authority said.