A failing dam is causing "extremely dangerous" flooding on a Puerto Rico river in the wake of Hurricane Maria, authorities say.
The National Weather Service (NWS) said the "imminent failure" of the Guajataca Dam was a "life-threatening situation".
More than 70,000 people live in the nearby areas of Isabela and Quebradillas.
At least 13 people have died since Maria ripped through Puerto Rico, knocking out power to the whole island.
The Guajataca Dam, at the northern end of Lake Guajataca in the north-west, began to show signs of failing at 14:10 local time (18:10 GMT) on Friday, operators said.
The NWS warned of flash flooding in Isabela and Quebradillas.
All 70,000 residents were initially told to flee but there are reports that the evacuation zone has since been narrowed.
The agency urged residents in the area to move to higher ground in an alert posted on its website. Many who live near the dam are being evacuated by buses.
Puerto Rico's governor has called Maria the worst hurricane in a century.
Ricardo Rossello says it could take months to restore electricity to all 3.4 million of the US island territory's residents.
Roofs were ripped off as strong winds battered Puerto Rico's capital city, San Juan.
Maria, a category three storm with maximum sustained winds of nearly 193km/h (120mph), was about 265km east-north-east of San Salvador, the National Hurricane Center said in its latest update at 09:00 GMT on Saturday.
"On the forecast track, Maria should move away from the Bahamas into the open waters of the western Atlantic today," the NHC said.
The hurricane has claimed more than 30 lives across the region, and is the second devastating storm to hit the Caribbean this hurricane season.
The first was category five Irma earlier in September.
Maria also caused widespread destruction on the small island of Dominica when it hit on Monday night, leaving at least 15 dead and 20 missing.
US President Donald Trump has pledged to visit Puerto Rico, saying it was "totally obliterated" by the storm.
Federal emergency aid has been made available.