Seventeen victims are from the Abaco islands and three from Grand Bahama, health minister Duane Sands told the Associated Press.He said three of the 20 victims died after being transported to a hospital in nearby New Providence.
Mr Sands says he is unable to release further details at this time because the government still is working to contact family members.
The sharp jump in the death toll comes after search and rescue teams fanned out across both islands as floodwaters receded and the weather cleared.
Dorian hit the Abaco islands as a catastrophic Category 5 storm on Sunday and then lashed Grand Bahama for a day and a half as a Category 4 storm, leaving widespread devastation.
"We planted those trees ourselves. Everything has a memory, you know," Greg Alem said as he walked over the ruins of his home.
"It's so, so sad. ... In the Bible there is a person called Job, and I feel like Job right now. He's lost everything, but his faith kept him strong."
Hurricane Dorian, back to a category three storm, has now begun raking the south-east US seaboard. It is threatening to inundate low-lying coasts from Georgia to south-west Virginia with a dangerous storm surge after its deadly mauling of the Bahamas
In South Carolina, USA more than 1,500 people have sought refuge in 28 shelters as authorities worried about the historic and vulnerable port city of Charleston.Dorian was centred overnight about 105 miles south of Charleston and moving north, just offshore.