Ian made another landfall on Friday in a second US state after tearing through Florida.
Ian, now a post-tropical cyclone, arrived near Georgetown, South Carolina, as a Category 1 hurricane with winds at 140 kilometres per hour, the National Hurricane Centre said. It weakened hours after landfall.
President Joe Biden pleaded with residents of South Carolina to listen to local officials' advice, as Ian is expected to bring "life-threatening flooding" to the state.
The storm has already left a deadly trail in its wake after slamming into the western coast of Florida on Wednesday and moving through the peninsula earlier this week.
At least 42 people were reported dead, CNN reported on Friday evening, citing local officials.
"We're just beginning to see the scale of that destruction," Mr Biden said in remarks from the White House.
As Hurricane Ian moved closer towards South Carolina, the streets of Charleston were eerily quiet as the city braced for what could be a heavy impact. Ian is expected to cause storm surges of up to two metres, which could cause heavy flooding in a city that is prone to it.
Even before Ian made landfall, water had already flooded several streets in the city’s historic quarter.
'The National' reports from the path of Hurricane Ian
Several three-story colonial and Georgian homes had been boarded up in an attempt to protect them from the raging winds, which have already caused havoc. Branches lay strewn about, blocking the way, while street lights flapped perilously.
Along the city’s battery, a defensive seawall designed to protect the area from coastal flooding, water and wind pounded the concrete structure as the occasional storm chaser stopped by.
Mr Biden approved an emergency declaration for South Carolina to provide federal funds for recovery efforts.
The president said he had been in contact with Governor Henry McMaster and remained in contact with Florida Governor Ron DeSantis.
Hurricane Ian interactive tracker
Ian weakened shortly after landfall, but is expected to continue to bring its wrath further inland across South Carolina and central North Carolina on Friday evening and Saturday.
North Carolina governor urged residents to prepare for the coming deluge.
“For North Carolinians, I want to be clear, this storm can still be dangerous, and even deadly,” he said during a news conference.
The storm is forecast to be downgraded to an extratropical low over North Carolina on Friday night or Saturday, and then expected to dissipate at the weekend, the NHC said.
Ian made landfall in Florida's Cayo Costa, west of Cape Coral and Fort Myers, on Wednesday afternoon as a Category 4 storm with maximum winds of 241 kph.
'Substantial loss of life'
Mr Biden said Hurricane Ian would likely be one of the worst in US history. The president earlier this week issued a sober warning that there could be a "substantial loss of life".
"It's not just a crisis for Florida. It's an American crisis" the president said.
The state's emergency management director, Kevin Guthrie, said there was one confirmed storm-related death in Polk County. He acknowledged Florida officials were investigating 12 deaths in Charlotte County and eight in Collier County that may be linked to Ian.
Mr Guthrie said 10,000 people remained unaccounted for, but noted many of them are likely in shelters or do not have access to power.
Authorities in Volusia County, Florida, first confirmed the death of a 72-year-old man from Deltona on Thursday morning. The sheriff's office said he was found in water in a canal behind his home early on Thursday morning.
The Florida Department of Law Enforcement had also confirmed a storm-related death of a 38-year-old man, who was killed after his car hydroplaned in the rain.
Mr DeSantis acknowledged that some people had died but did not provide a specific figure, saying official confirmation was still needed.
“We fully expect to have mortality from this hurricane,” he said during a news conference on Thursday evening.
Three others were reported dead in Cuba after Ian struck the island on Tuesday.
Recovery efforts continue
Search and rescue operations continued on Friday afternoon and Mr DeSantis said rescue teams had gone door to door to more than 3,000 homes in the state.
“There’s really been a Herculean effort,” he said during a news conference in Tallahassee.
The National Guard and coastguard are also using helicopters to rescue people on barrier islands, the governor's office said.
And after announcing a major disaster declaration earlier in the day, Mr Biden directed federal funds that would assist in rebuilding homes and covering insurance losses.
The storm is expected to be one of the costliest in history. Estimated losses from Hurricane Ian's damage are between $28-$47 billion, financial services company CoreLogic predicted.
Mr Biden said "it's going to take months, years" to rebuild the devastated areas.
Bird's eye views of a state laid waste by Hurricane Ian
Florida homes still without power
Nearly 2 million Florida homes and businesses were still left without electricity on Friday, said PowerOutage.us.
The storm had wiped out power for nearly every building in three counties.
“Lee and Charlotte are basically off the grid at this point,” Mr DeSantis said during a news conference on Thursday morning.
Those in the hardest-hit areas of the storm were unable to call for help because of the outages.
Fema administrator Deanna Criswell said power-restoration teams were making their way towards affected communities to bring them back online.
