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 is 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.
Operators of the Guajataca Dam said the structure, at the northern end of Lake Guajataca in northwest Puerto Rico, began to show signs of failing at 14:10 local time (18:10 GMT).
It sparked a flash flood emergency for Isabela and Quebradillas municipalities, the NWS said in a series of tweets.
The agency urged residents in the area to “move to higher ground now” in an alert posted on its website.
Many who live near the dam are being evacuated by buses.
Maria, a category three storm, is now moving away from the Turks and Caicos Islands.
It is expected to head to the northeast and east of the Bahamas over the weekend, forecasters say.
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 140mph (225km/h) winds battered Puerto Rico’s capital city, San Juan.
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.
He has yet to declare the island a disaster area, but has made federal emergency aid available.