People prepare for the arrival of Hurricane Fiona in Puerto Rico
A man surveys his water front neighbourhood as Hurricane Fiona and its heavy rains approaches in Guayanilla, Puerto Rico September 18, 2022. Reuters

Hurricane Fiona was barreling toward Puerto Rico on Sunday, threatening to slam the U.S. territory with life-threatening flash floods and mudslides, while already knocking out a third of the island's fragile power grid.

The storm was about 80 km south of the city of Ponce with maximum sustained winds near 130 kilometers per hour, clearing the threshold for Category 1 hurricane strength, the National Hurricane Center said in an update at 11:00 a.m. (1500 GMT).

Some 551,333 utility customers were without electricity out of 1.47 million on the island of 3.3 million people, the Puerto Rico government said. Ports have been closed and flights out of the main international airport canceled, with Fiona set to move near or over Puerto Rico as early as Sunday afternoon.

Torrential rains and mudslides were also forecast for the Dominican Republic as the storm progresses northwestward. The Turks and Caicos Islands could face tropical storm conditions on Tuesday, the NHC said.

"Additional strengthening is forecast during the next 48 hours while Fiona moves near Puerto Rico, the Dominican Republic and over the southwestern Atlantic," it said.

U.S. President Joe Biden approved an emergency declaration for Puerto Rico on Sunday, a move that authorizes the Federal Emergency Management Agency to coordinate disaster relief and provide emergency protective measures.

The rains have increased in intensity since Sunday morning, along with strong wind gusts, residents said.

"It has been raining heavily since 10:00 a.m. At the moment, we still have electricity service," said Kimberly Ortiz, who lives in Ciales, a town in the center of the island. "In Ciales, we have felt some wind gusts."

A wide swathe of Puerto Rico was forecast to see 12 to 16 inches (30 to 40 cm) of rain while parts could be hit by up to 25 inches (63.5 cm), according to the NHC.

"These rains will produce life-threatening flash flooding and urban flooding across Puerto Rico and the eastern Dominican Republic, along with mudslides and landslides in areas of higher terrain," the NHC said.

Puerto Rico's grid remains fragile after Hurricane Maria in September 2017 caused the largest blackout in U.S. history. In that category 5 storm, 1.5 million customers lost electricity with 80% of power lines knocked out.

Authorities have opened more than 100 shelters and closed beaches and casinos, and residents were urged to seek shelter.

There has been one death reported so far tied to Fiona, in the French Caribbean island of Guadeloupe. Authorities said one man as found dead on Saturday after his house was swept away by floods there. France will recognize a state of natural disaster for Guadeloupe, President Emmanuel Macron said on Twitter on Sunday.