Thousands remained cut off by severe flooding across Vermont, New Jersey and upstate New York Wednesday in the lingering aftermath of Hurricane Irene, which has killed nearly 50 people.

US President Barack Obama signed a declaration that a "major disaster" existed in parts of North Carolina and New York, paving the way for federal aid to areas hit by massive flooding and widespread power outages.

Emergency provisions had to be airlifted on Tuesday to dozens of communities stranded by floodwaters as torrential weekend rains dumped by the massive storm system washed away roads and sent rivers cascading over their banks.

Although the much-hyped direct-hit on New York failed to translate into major damage or casualties in America's most populous city, heavy rain in places like the Catskill Mountains proved a ticking disaster time-bomb.

Three days after the storm's passage, marooned families were still waiting anxiously for the national guard and firefighters to bring food and water to towns swamped by the floodwaters.

In other places, rescuers have been ferrying thousands of people -- including the elderly, children and babies -- to safety in rubber motorboats.

The main highway to Wilmington, Vermont was clogged with mud and Irene had turned other roads into deathtrap chasms after dumping two months worth of rain (8.3 inches, 21 centimeters) in less than a day.

"The problem is inaccessibility," emergency operations supervisor Dave Miller told AFP as teams struggled to pull trucks out of the sludge and remove fallen trees that had perilously dragged down power lines.

The drastic situation was mirrored in parts of New Jersey and upstate New York, where schools and community centers turned into makeshift Red Cross emergency shelters were nearing full capacity.

In Paterson, New Jersey, teams rescued and evacuated people non-stop on Tuesday under thankfully blue skies after the Passaic River crested 13 feet (four meters) above flood stage, its highest level since 1903.

New Jersey state lawmaker Scott Rumana, touring stricken areas with Governor Chris Christie, said it was "unquestionably the biggest flooding event in our lifetimes."

Millions of Americans remained without electricity, many farther south in states like Virginia and North Carolina, where Irene's winds were strongest as the storm barreled up the eastern seaboard on Saturday and Sunday.

Vermont, a mountainous state crisscrossed by numerous streams and rivers, saw several towns completely cut off by the floods.

"There are currently 13 communities that are unreachable by vehicle due to road damage," said a statement from Vermont Emergency Management.

"There are more than 200 roads that are still impassable statewide and all 500 road workers from the Agency of Transportation are on the street today working on repairs."

Dramatic television pictures from New Jersey, New York and Vermont showed flash floods sweeping through towns and fields turned to lakes where rivers had burst their banks.

Homeland Security Secretary Janet Napolitano and Agriculture Secretary Tom Vilsack visited Virginia and North Carolina on Tuesday, while top disaster official Craig Fugate went to Burlington, Vermont.

"What I saw in North Carolina was really quite startling and quite significant in terms of agricultural damage," Vilsack told a conference call with journalists afterwards.

"Corn fields that were destroyed, cotton fields where cotton was now lying on the ground, damage to soya bean as well as tobacco fields in North Carolina as well as Virginia... significant damage to the tomato industry."

Research firms have estimated that Irene could cause up to $7 billion in damage, but that does not factor in the longer-term effect on farmers.

Officials have reported at least 43 deaths across 11 states, including eight in New York, seven in New Jersey and six in North Carolina, where Irene made landfall Saturday with winds upwards of 85 miles (140 kilometers) an hour.

The hurricane was already responsible for at least five deaths in the Caribbean before it struck the United States, and is being blamed for a 49th fatality in Canada, where the storm finally petered out on Tuesday.

More trouble was on the way though as Tropical Storm Katia formed in the Atlantic, forecast to become a category 3 hurricane by Saturday or Sunday with winds topping 120 miles per hour.