The second major hurricane to rage through the Caribbean this month, Maria has killed at least 32 people and devastated several small islands, including St. Croix in the US Virgin Islands and Dominica.
At least 15 people were killed in Puerto Rico, the island's El Nuevo Día newspaper reported.
Among those were eight people who drowned in Toa Baja, about 20 miles (32 km) west of San Juan, Mayor Bernardo Márquez told the newspaper, adding the names of those who died have not been released and the causes of death had to be confirmed through autopsies.
More than 4,000 people had been rescued from flooded areas of Toa Baja, the mayor was quoted as saying.
Three elderly sisters were killed by a mudslide on Wednesday in the mountainous central municipality of Utuado, El Nuevo Día said, citing relatives and the mayor of Utuado.
Maria, headed toward the Turks and Caicos islands, is now a Category 3 hurricane on the five-step Saffir-Simpson scale, with sustained winds of up to 125 miles per hour (205 km per hour). It was 65 miles (110 kms) east-southeast of Grand Turk Island as of 11 p.m. EDT (0300 GMT), the US National Hurricane Center said.
Maria was expected to bring as much as 40 inches (102 cms) of rain to Puerto Rico and 8 to 16 inches (20 to 40 cm) to the Turks and Caicos, which could cause flash floods and mudslides, the NHC said.
Maria's strength was not expected to change during the next few days, the center said. The storm looked unlikely to hit the continental United States but its storm swells will reach the US southeastern coast from Friday, the NHC said.
Officials in Puerto Rico were assessing the damage after Maria slammed the island on Wednesday with winds of up to 155 mph (250 kph). Ranked a Category 4 storm when it made landfall, it was the strongest hurricane to hit the island in nearly 90 years.
US. President Donald Trump told reporters the island had been "totally obliterated" and that he planned to visit.
Puerto Rico Governor Ricardo Rossello imposed a dusk-to-dawn curfew through Saturday on the island of 3.4 million people.
"If it is not an emergency situation, people should remain in their homes or shelters," he was quoted as saying by El Nuevo Dia.
Maria struck Dominica as a Category 5 storm on Monday night, damaging about 95 percent of the roofs on the island, the UN Office for the Coordination of Humanitarian Affairs said. At least 14 people died, CNN quoted Charles Jong, a spokesman for the Dominica prime minister's office, as saying.
Two people were killed in the French territory of Guadeloupe and one person on the US Virgin Islands.