US President Joe Biden is visiting Louisiana on Friday to survey the damage caused by Hurricane Ida, which made landfall in the southern US state on Sunday.
The visit comes days after the storm's remnants struck the US north-east and mid-Atlantic regions, which suffered "historic" flooding and devastating tornadoes.
Catastrophic flooding has left at least 46 dead in five states, Reuters reported on Friday.
In a televised address on Thursday, US President Joe Biden pledged robust help for the north-east as well as the Gulf Coast region, saying extreme events like Ida are a reminder that the climate crisis is real.
“These extreme storms, and the climate crisis, are here,” Mr Biden said. “We must be better prepared. We need to act.”
Mr Biden spoke with New York Governor Kathy Hochul and New Jersey Governor Phil Murphy, and said that he planned to speak with Pennsylvania Governor Tom Wolf.
New York, New Jersey and Connecticut have all declared states of emergency.
The president added he would order the use of military drones and satellites to speed up the “complicated and really dangerous” repair work.
At least 15 people have died in New York City, including 11 who were trapped in basements, the New York City Police Department said.
Floodwaters and a falling tree also took lives in Maryland, Pennsylvania and Connecticut.
At least 25 others were killed in New Jersey, the Associated Press reported.
The chief of the NYPD’s community affairs bureau, Jeffrey Maddrey, said officers were going to door-to-door searching for people who may have been trapped or killed.
“We did not know that between 8.50pm and 9.50pm last night that the heavens would literally open up and bring Niagara Falls level of water to the streets of New York,” Ms Hochul said at a briefing in Queens on Thursday.
In an earlier phone call, Mr Biden “offered any assistance the state needs,” she said.
Ms Hochul filed for federal support in the counties that were hit hardest by the torrential rains and flooding.
New York City Mayor Bill de Blasio said weather projections had failed to predict such a cataclysmic downpour.
“We’re getting from the very best experts projections that then are made a mockery of in a matter of minutes,” he said.
“That turned into the biggest single hour of rainfall in New York City history with almost no warning.”
New York and New Jersey leaders addressed the urgency of addressing infrastructure, something that Mr Biden noted in his speech. The president said he will further press Congress to pass his nearly $1 trillion infrastructure bill to improve roads, bridges, the electric grid and sewer systems.
In Pennsylvania, record flooding inundated homes, swamped cars and disrupted rail service in the Philadelphia area.
Emergency workers in the county completed more than 450 water rescues.
On Wednesday night Mr de Blasio declared a state of emergency due to what he called a “historic weather event” with record rain across the city leading to flooding and dangerous conditions on the road, Reuters reported.
The city's subway lines were suspended late on Wednesday as the remnants of Hurricane Ida brought drenching rain and the threat of flash floods and tornadoes to parts of the northern mid-Atlantic, CNN reported.
At least one person was killed as the flooding hit the New Jersey city of Passaic, Mayor Hector Lora told CNN.
NBC New York reported that one more person had died in New Jersey and eight had died in New York City, including a 2-year-old boy. Local media reported that people were trapped in their basements as the storm sent water surging through the city.
Mr Lora said the body of a man in his 70s was retrieved from the floodwaters. The vehicle the man was riding in was swept away by the water and firefighters were swept under it, preventing them from reaching him, CNN said.
“Take shelter NOW. Flying debris will be dangerous to those caught without shelter. Move to a lower floor and stay away from windows,” New York City's emergency notification body said on Twitter.
The city issued a travel ban right before 1am that was in effect until 5am on Thursday.
“All non-emergency vehicles must be off NYC streets and highways,” the emergency management office said.
Late on Wednesday, the Metropolitan Transportation Authority told customers in an email that “service is extremely limited, if not even suspended, because of heavy rainfall and flooding across the region”.
The city's subway said 18 lines had been suspended.
At least five flash-flood emergencies were issued on Wednesday evening by the National Weather Service, stretching from west of Philadelphia through northern New Jersey.
Mr Murphy declared a state of emergency and said the storm caused 57,519 power cuts across the state and “these numbers are climbing".
Newark, LaGuardia and JFK airports cancelled hundreds of flights as flooding closed major roads across boroughs of the metropolis including Manhattan, The Bronx and Queens.
Almost all of New Jersey's rail services were suspended.
The city earlier issued a rare flash-flood emergency warning, urging residents to move to higher ground.
“Significant and life-threatening flash flooding is likely from the mid-Atlantic into southern New England,” the National Weather Service said in a bulletin, and added that seven to 20 centimetres of rain could drench the region through Thursday.
In Annapolis, 50 kilometres from the US capital, a tornado ripped up trees and toppled electric poles.
A 19-year-old man was killed in Maryland and another person went missing after a building was flooded on Wednesday, bringing the death toll from Ida to seven.
The hurricane, which hit Louisiana on Sunday as a Category 4, is now a post tropical cyclone, the National Hurricane Centre said.
Ida is expected to continue steaming north and bring heavy rainfall on Thursday to New England, which was hit by a rare tropical storm in late August.
Mr Biden is due to travel on Friday to Louisiana, where Ida destroyed buildings and left more than a million homes without power.
Hurricanes are common in the southern US, but scientists have warned of a rise in cyclone activity as the ocean surface warms due to climate change, posing an increasing threat to the world's coastal communities.