More than 50 people are confirmed dead and almost 3.5 million are without power on Saturday, after strong winds and torrential rain from Hurricane Helene wreaked unprecedented havoc across large swathes of the south-eastern United States.

Historic flooding continued over parts of the southern Appalachians on Saturday, as first responders worked to reach stranded communities in trying conditions while local authorities began to assess the scale of the damage and displacement.

“It looks like a bomb went off,” said Georgia’s governor, Brian Kemp, after surveying the damage from the air on Saturday.

“To say this caught us off-guard would be an understatement,” said Quentin Miller, sheriff of Buncombe county, North Carolina, where part of Asheville is underwater and multiple cell towers remain down, hampering rescue and recovery efforts. Emergency services have declined to confirm the number of fatalities in the county until communication outages can be restored and next of kin informed.

In a statement also on Saturday, Joe Biden said that the head of the Federal Emergency Management Agency (Fema), Deanna Criswell, is travelling throughout the south-east to assess the damage alongside other state and local officials.

“I am deeply saddened by the loss of life and devastation caused by Hurricane Helene across the south-east … My administration is in constant contact with state and local officials to ensure communities have the support and resources they need,” he said. “We’re not going to walk away. We’re not going to give up.”

Helene made landfall late on Thursday in Florida’s Big Bend region as a category 4 hurricane, pummeling the peninsula with winds of 140mph (225km/h). It weakened into a tropical storm, moving quickly through Georgia, the Carolinas and Tennessee, uprooting trees, blowing roofs off homes, sweeping away cars, testing dams and flooding rivers – leaving entire communities without escape as landslides and flooding struck.

A combination of strong winds, heavy rain, flooding and tornadoes that followed in the path of Helene have likely caused billions of dollars in damage, with entire downtowns, highways, and large numbers of homes and businesses ruined.

Jonathan Porter, chief meteorologist at AccuWeather, estimated the damage from the storm to cost between $95bn and $110bn, potentially making this one of the most expensive storms in modern US history.

According to a tally by the Associated Press, Helene has so far caused at least 52 deaths in Florida, Georgia, North Carolina, South Carolina and Virginia, and include firefighters, a woman and her one-month-old twins, and an 89-year-old woman whose house was struck by a falling tree. Scores of people including multiple children remain in hospital with serious injuries.

By midday on Saturday, just over a million homes and businesses remained without power in South Carolina, with 750,000 also in the dark across Georgia and 600,000 in North Carolina, according to PowerOutage. Florida, Virginia, Ohio and Kentucky are also badly affected, as well as tens of thousands of people in Indiana, West Virginia and Tennessee.

The threat of further deaths and destruction is ongoing but Helene has weakened to a post-tropical cyclone with the risk of additional heavy rainfall waning as it moves across the Tennessee valley, according to the National Hurricane Center.

Scores of dramatic water evacuations and rescues were carried out on Friday as unprecedented heavy rain strained dams and rivers.

In North Carolina, a lake featured in the movie Dirty Dancing overtopped a dam and surrounding neighborhoods were evacuated as a precaution. Parts of western North Carolina were largely cut off by landslides and flooding that forced the closure of major roads.

In rural Unicoi county in east Tennessee, dozens of patients and staff were rescued by helicopter from the roof of a hospital that was surrounded by water from a flooded river.

Meanwhile in Mexico, at least 22 people were confirmed dead on Saturday after Tropical Storm John made its second landfall and flooded the southern resort city of Acapulco – which still hasn’t recovered from Hurricane Otis last October.

John first made landfall as a category 3 hurricane farther north in the state of Michoacán, weakening inland, and then gathering strength again over the ocean before making landfall in Acapulco. Local authorities pleaded for help from boat owners after a year’s worth of rain that pounded the coastal mountains triggered landslides and severe flooding in Acapulco and elsewhere.

Global heating, which is driven by burning fossil fuels, is supercharging tropical storms by generating conditions that enable rapid intensification, sometimes within hours, and bring a heightened risk of flooding.

Atlantic storms have become deadlier as the planet warms – and are disproportionately killing people of color in the US, according to one landmark study. About 20,000 excess deaths – the numbers of observed rather than expected deaths – occurred in the immediate aftermath of 179 named storms and hurricanes which struck the US mainland between 1988 and 2019.

The National Hurricane Center is currently monitoring two more storms that are moving through the Atlantic – Tropical Storm Joyce and Hurricane Issac, which is gathering strength.