Bahamas Prime Minister Hubert Minnis has lamented the "generational devastation" wrought by Hurricane Dorian, with the death toll rising to at least 20.
The storm has been downgraded to Category 2 as it bears down on North and South Carolina on the US east coast.
Mr Minnis confirmed the storm's death toll had risen to at least 20 during a news conference in which he described the unthinkable damage that parts of his island nation had sustained.
The US Coast Guard and Britain's Royal Navy airlifted survivors and ferried in emergency supplies as floodwaters receded in the Bahamas, while the centre of the strengthening storm continued toward South Carolina.
The Miami-based National Hurricane Centre (NHC) said Dorian, which has already wrought major damage on its course through the Atlantic, was packing maximum sustained winds of 185km/h.
The United Nations said 70,000 people on Grand Bahama and Abaco islands were in "immediate need" of aid while the prime minister confirmed the new victim count.
He also issued a warning to looters, saying they will be prosecuted "to the fullest extent of the law" and announced the deployment of additional police and defence force officers.
Shelter, safe drinking water, food and medicine were urgently needed for some 50,000 people on Grand Bahama and between 15,000 and 20,000 on Abaco, UN emergency relief coordinator Mark Lowcock said after a meeting with Mr Minnis.
"Speed is of the essence," Red Cross official Stephen McAndrew said of rescue operations on the two northernmost islands in the Bahamas archipelago which were pummeled by one of the strongest Atlantic storms on record.
People on Grand Bahama were using jet skis and boats to pluck victims from homes flooded and pulverised by heavy rain and lashing winds from the monster storm.
US and British helicopters were conducting medical evacuations, aerial assessments to help coordinate relief efforts, and reconnaissance flights to assess damage.
US President Donald Trump spoke by telephone to the Bahamian leader and pledged US assistance, the White House said.
"A big section of the Bahamas was hit like few people have seen before," Mr Trump said. "They need a big hand."
Aerial footage showed scenes of catastrophic damage in Abaco with hundreds of homes missing roofs, cars submerged or overturned, widespread flooding and boats reduced to matchwood.
As rescue efforts ramped up, Dorian was expected to "approach the coast of South Carolina tonight, move near or over the coast of South Carolina on Thursday, and then move near or over the coast of North Carolina Thursday night and Friday," the NHC said.
It had, however, left Florida largely unscathed.
"We got lucky in Florida, very, very lucky indeed," Mr Trump said.
Life-threatening storm surge with significant coastal flooding was expected along large parts of the southeast and mid-Atlantic coast over the following days, the NHC said.
The centre predicted the Carolinas could be hit with dangerous storm surge of up to 2m and up to 30cm of rain.
"We are very well prepared," Mr Trump said.
The US Coast Guard said it had rescued 61 people including 19 injured patients from the Marsh Harbour clinic on Abaco island who were flown to Nassau on Tuesday.
USAID, the US relief agency, said meanwhile it was airlifting supplies such as plastic sheeting for shelter, hygiene kits and water from Miami.
Dorian has dumped as much as 76cm of rain on the Bahamas.
According to a NHC bulletin early this morning, Dorian was located about 170km south of Charleston, South Carolina and was moving north at 11km/h.
