It was a miracle that the death toll was not in the hundreds, PM Roosevelt Skerrit tearfully told a local television station.
Hurricane Maria ripped through Dominica as a category four storm late on Monday, damaging hundreds of homes.
The storm later devastated Puerto Rico, leaving the whole island without power.
Mr Skerrit said he has spent the last 24 hours surveying by air the destruction caused by the powerful storm.
“It has been brutal,” he said on Thursday on the nearby island of Antigua. “We’ve never seen such destruction.”
Homes have been flattened, schools have been destroyed, telecommunications have been cut off and the island’s main hospital is still without electricity, he said.
The hospital’s generators had to be set aside because of flooding and officials were still working to see if they could be turned on.
He added the island is “going to need all the help the world has to offer”.
Meanwhile, authorities in Puerto Rico are urging residents to seek higher ground ahead of flash flooding – with up to 30 inches (76cm) more rain predicted by Saturday – in the aftermath of Maria.
The island’s Governor Ricardo Rossello described the hurricane as “the most devastating storm in a century” and said that Maria had hit the island’s electricity grid so badly that it could take months to restore power.
Images shared on social media show roofs being stripped away as winds as strong as 140 mph (225 km/h) whipped trees and power lines in Puerto Rico’s capital city, San Juan.
Maria, now a category three storm, is moving off the northern coast of the Dominican Republic and heading towards the Turks and Caicos Islands.
The storm has claimed at least 17 lives so far across the Caribbean, with many others missing, officials say. Two people were killed in the French territory of Guadeloupe.
US President Donald Trump said the storm had “totally obliterated” the US territory, and pledged to visit Puerto Rico.
He has yet to declare the island a disaster area but has made federal emergency aid available.
It is the second devastating storm to hit the Caribbean this hurricane season – the first being category five Irma earlier in September.
Irma was one of the most powerful Atlantic storms on record, killing at least 84 people in the Caribbean and US.
The hurricane followed Harvey, which killed more than 80 people after it barrelled through Texas last month.