Maria has claimed at least 18 lives across the Caribbean since striking Dominica on Monday. The storm is expected to hit the Turks and Caicos Islands and the Bahamas by late Thursday and early Friday.
Puerto Rico officials were weighing the damage inflicted by Hurricane Maria on Thursday, after the devastating storm destroyed buildings and knocked out power across the densely populated island.
Hurricane Maria hit Puerto Rico on Wednesday with winds of up to 155 miles per hour (250 kph). Ranked as a Category 4 storm when it made landfall, Maria was the strongest storm to strike the island in almost 90 years.
Read more: Hurricane Maria hits Puerto Rico
In the capital, San Juan, historic forts and buildings dating back to the Spanish colonial era were destroyed, leaving nothing but debris.
Puerto Rico Governor Ricardo Rossello said one death had been reported so far, although the full extent of the storm was still being assessed. The government could not yet provide an estimate on how many home and businesses were destroyed in Maria's onslaught. Communication channels have been almost impossible to maintain, with roads destroyed and telephone towers knocked out across the island.
US President Donald Trump said on Thursday that the hurricane had "obliterated" Puerto Rico and declared the island a disaster zone in a move that frees up federal relief funding.
Rossello warned residents to expect rainfall over the coming days, which could lead to devastating floods and mudslides.
After brushing the Dominican Republic on Thursday, Hurricane Maria is expected to head towards the Turks and Caicos Islands and the Bahamas by late Thursday and early Friday. From there, it is forecast to veer back towards the Atlantic, avoiding the US mainland.
Power and phone networks knocked out
Ajit Pai, the chairman of the US Federal Communications Commission (FCC) on Thursday reported that more than 95 percent of the Puerto Rico's wireless phone sites were not working. Maria, he said, "has had a catastrophic impact on Puerto Rico's communications networks" and that "getting Puerto Rico's communications networks up and running will be a challenging process, particularly given the power outages throughout the island."
The head of Puerto Rico's electricity board, Ricardo Ramos, said the island's electricity board had been completely destroyed and warned it could take months before power was fully restored.
Speaking to CNN, Ramos said: "I guess it's a good time for dads to buy a glove and ball and change the way you entertain your children and the way you are going to go to school and the way you are going to cook for gas stoves other than electric."
Many of the worst hit areas in Puerto Rico were under water, with more heavy rainfall expected in the coming days.
Dominican Republic suffers flooding
Maria also brought down trees and power lines in the Dominican Republic, as it brushed past country's coast on Thursday after being downgraded to a Category 3 storm.
However, according to the Joel Santos, president of the Dominican Republic's hotel industry, none of the islands' tourism infrastructure was damaged in the storm.
Nevertheless, the Dominican Republic's northern areas were without power throughout the day and the country could still be hit by storm surges. Forecasters on Thursday warned of "large and destructive waves" reaching between one and two hitting the coast, while the northern and eastern parts were expected to receive 20 to 40 centimeters of rain on Saturday.
Dominica buries its dead
Maria claimed its highest death toll on the island of Dominica, where at least 15 people were killed when it hit the island as a Category 5 storm on Monday.
Around 95 percent of homes in Dominica were damaged in the onslaught. By Thursday the island was still without electricity or drinking water, Dominica's Prime Minister Roosevelt Skerrit reported.
"It has been brutal, said Skerrit, who himself had to be rescued after Maria blew off the roof off his residence. "I saw almost complete devastation. It is worse than a war zone."
Two more deaths were reported on the French island of Guadeloupe.
dm/rc (AP, Reuters, AFP, dpa)