A dam has failed and caused "extremely dangerous" flooding on Puerto Rico's Guajataca river in the wake of Hurricane Maria, authorities say.
The National Weather Service (NWS) said buses were "currently evacuating people from the area as quickly as they can".
At least 13 people have died on the US territory since Maria ripped through Puerto Rico, devastating homes and knocking out the island's electricity.
The island's governor has called it the worst storm in a century.
Operators of the Guajataca Dam said the dam, located at the northern end of Lake Guajataca in northwest Puerto Rico, failed at 14:10 local time (18:10 GMT).
The agency urged residents in the area to "move to higher ground now" in an alert posted on its website.
"This is an extremely dangerous and life-threatening situation.Do not attempt to travel unless you are fleeing an area subject to flooding or under an evacuation order," the alert said.
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Hurricane Maria, a category three storm, is now moving away from the Turks and Caicos Islands and is expected to head to the northeast and east of the Bahamas over the weekend, forecasters say.
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It has claimed more than 30 lives across the region, and is the second devastating storm to hit the Caribbean this hurricane season - the first being category five Irma earlier in September.
Maria caused widespread destruction on the small island of Dominica, where it flattened homes, destroyed buildings and cut off telecommunications when it hit on Monday night.
At least 15 people have died and 20 others are missing on Dominica after the tempest, according to the Caribbean island's Prime Minister Roosevelt Skerrit.
Puerto Rico's Governor Ricardo Rossello described the hurricane as "the most devastating storm in a century".
He said Maria had hit the island's electricity grid so badly that it could take months to restore power.
Images shared on social media show roofs being stripped away as winds as strong as 140mph (225km/h) whipped trees and power lines in Puerto Rico's capital city, San Juan.
US President Donald Trump said the storm had "totally obliterated" the US territory, and pledged to visit Puerto Rico.
He has yet to declare the island a disaster area but has made federal emergency aid available....