The head of the US disaster relief agency has called Hurricane Helene, which has killed nearly 100 people, a “true multi-state event” that caused “significant infrastructure damage” and had been made worse because of global heating.

The storm killed at least 91 people, according to state and local officials in South Carolina, Florida, Georgia, North Carolina and Virginia. Officials feared more bodies would be discovered.

“This is going to be a really complicated recovery in each of the five states” of Florida, Georgia, North Carolina, South Carolina and Tennessee, said the Federal Emergency Management Agency (Fema) administrator, Deanne Criswell.

She noted that a 15ft storm surge hit Florida’s Taylor county, where Helene came ashore as a category 4 hurricane late Thursday with winds of 140mph (225km/h), and pointed out that areas of western North Carolina, where search and rescue operations are continuing, recorded 29in (74cm) of rain when the storm stalled over the region.

“This is historic flooding up in North Carolina,” Criswell told the CBS show Face the Nation on Sunday. “I don’t know that anybody could be fully prepared for the amount of flooding and landslides they are having right now.”

Kamala Harris said the Joe Biden administration had approved emergency declarations for Alabama, Florida, Georgia, North Carolina, South Carolina and Tennessee, “making resources and funding available to maximize our coordinated response efforts at the local, state and federal levels”.

The White House said on Sunday that Biden intended to travel this week to communities affected by Hurricane Helene as soon as it would not disrupt the emergency response.

Biden spoke with Georgia governor Brian Kemp and North Carolina governor Roy Cooper on Sunday evening to get updates on response and recovery efforts, and to promise continued support to affected communities.

A North Carolina county that includes the mountain city of Asheville reported 30 people killed due to the storm. As many as 1,000 people remain unaccounted for in Buncombe county in the Appalachian mountains, where the hurricane caused catastrophic flooding and mudslides in the Asheville region, cutting off most communication and making the roads impassable.

Supplies were being airlifted to the region around the isolated city. Buncombe county’s manager, Avril Pinder, pledged that she would have food and water into Asheville – known for its arts, culture and natural attractions – by Monday.

More than 150 search and rescue operations were under way in the state. Cooper’s office said on Sunday that “people are desperate for help”. “Even as the rain and winds have subsided, the challenge for people there increases.”

Quentin Miller, the Buncombe county sheriff, said: “To say this caught us off-guard would be an understatement.”

Pinder called the storm “Buncombe county’s own Hurricane Katrina”, and officials said communication systems had been disrupted, with no cellphone service expected in the region for at least “several days”. At least 23 are dead in South Carolina, including two firefighters. In Georgia, at least 17 people have died, two of them killed by a tornado in Alamo, according to a spokesperson for Kemp.

Kemp said on Saturday that it “looks like a bomb went off” after viewing splintered homes and debris-covered highways from the air.