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SAN JUAN, Puerto Rico — Hurricane Maria is likely to have "destroyed" Puerto Rico, the island's emergency director said Wednesday after the monster storm ripped roofs off buildings and flooded homes.

Intense flooding was reported across the economically strained U.S. territory, particularly in San Juan, the capital, where many residential streets looked like rivers.

The National Weather Service issued a flash flood warning for the entire island shortly after 12:30 a.m. ET.

Puerto Rico is now completely under a Flash Flood Warning. If possible, move to higher ground NOW! #prwx pic.twitter.com/TOrVnXaKOR — NWS San Juan (@NWSSanJuan) September 21, 2017

Yennifer Álvarez Jaimes, Gov. Ricardo Rosselló's press secretary, told NBC News that all power across the island was knocked out.

"Once we're able to go outside, we're going to find our island destroyed," Emergency Management Director Abner Gómez Cortés said at a news briefing. Rosselló imposed a 6 p.m.-to-6 a.m. curfew, citing flood warnings and the importance of keeping streets clear for repair and rescue teams.

San Juan Mayor Carmen Yulín Cruz told MSNBC that the devastation in the capital was unlike any she had ever seen.

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"The San Juan that we knew yesterday is no longer there," Yulín said, adding: "We're looking at four to six months without electricity" in Puerto Rico, home to nearly 3.5 million people.

Maria, the strongest storm to hit Puerto Rico since 1928, had maximum sustained winds of 155 mph when it made landfall as a Category 4 storm near the town of Yabucoa just after 6 a.m. ET, the National Hurricane Center said.

After weakening, Maria strengthened to a Category 3 hurricane early Thursday, it added. By 2 a.m. ET, it was about 55 miles north of Punta Cana, Dominican Republic.

Tropical storm conditions were expected to continue on Puerto Rico through the night, but hurricane warnings for the island were lifted late Wednesday as Maria moved away from its the northwest coast.

"The wind threat has decreased," the hurricane center said, but the threat of rain-gorged floods remains "devastating to catastrophic," it said. Airports in San Juan, Aguadilla and Ponce were ordered closed until Friday at the earliest because of flooding and debris, authorities said.

"Extreme rainfall flooding may prompt numerous evacuations and rescues," the agency said. "Rivers and tributaries may overwhelmingly overflow their banks in many places with deep moving water."

Rosanna Cerezo, a lawyer and radio host in metro San Juan, said the city was deluged. It sounded as though bombs were going off when the wind toppled trees around her house, she said.

Along the beachfront, she said, cement structures had been wrenched from their foundations as islanders scrambled for refuge.

"Rooftops collapsed, windows shattered," Cerezo said in a text message. "People are huddled in hallways, closets, bathrooms."

Maria's next move was expected to take it near the coasts of the Dominican Republic and the Turks & Caicos islands, which were under hurricane warnings, and then begin drifting more northwestward by Friday.

Forecasters said it remained too early to know how close Maria will move to the U.S. mainland, but Domenica Davis, a meteorologist for The Weather Channel, said, "It looks like it will stay in the open waters of the Atlantic."

Puerto Rico was already struggling to dig itself out of a historic financial crisis. Maria could destroy any progress the territory has made under a year-old economic rehab plan ─ and set it back further.

Maria was a Category 5 hurricane — the strongest there is — when it hit the Caribbean on Monday night, killing at least seven people on the island of Dominica and one person on Guadeloupe.

Gadi Schwartz reported from Puerto Rico. Alex Johnson reported from Los Angeles. Daniel Arkin, Daniella Silva and Sandra Lilley reported from New York. Yuliya Talmazan reported from London.