Abner Gómez, head of the disaster management agency, said Hurricane Maria had damaged “everything in its path”.
He said that none of the customers of Puerto Rico’s Electric Power Authority had any electricity.
Maria weakened to a category 3 storm with winds of 115 mph as it moved across the island.
“The information we received is not encouraging,” Abner Gómez told a briefing, urging people to stay in their homes.
Earlier, Puerto Rico Governor Ricardo Rossello asked President Donald Trump to declare the island a disaster area after the storm unleashed heavy flooding and life-threatening winds.
The governor said major damage was inevitable, although 500 shelters had been established to protect people.
Hurricane Maria has already left seven people dead on the island of Dominica, which was badly affected on September 18.
Aerial footage shows flattened houses and the death toll on Dominica is likely to rise, with details remaining scant as communication links are down.
Images shared on social media show roofs being stripped away as winds as strong as 140 mph whipped trees and power lines in Puerto Rico’s capital city, San Juan.
Puerto Rico is facing $73 billionn in public debt and the damage left by Maria could exacerbate its financial crisis. Declaring a disaster would mean the island could receive more federal assistance in its recovery.
According to the US National Hurricane Center (NHC), Maria made landfall in Yabucoa in the east of Puerto Rico on September 20 as a category 4 storm. It was the first category 4 hurricane to directly hit Puerto Rico since 1932.
Hours earlier, Maria barreled through the US Virgin Islands’ St Croix as a category 5 storm, sustaining winds of up to 175mph.
There was flooding in the French territory of Guadeloupe, where one person was killed by a falling tree and another died on the seafront. At least two others were missing after their ship sank near Desirade, the easternmost island in the archipelago.
Hurricane Maria is the second devastating storm to hit the Caribbean this season – the first being category 5 Irma earlier in September. Maria began moving roughly along the same track as Irma.
Officials in Puerto Rico feared the debris left by Irma could prove extremely dangerous in the high winds.
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