Thousands of people downstream of a failing dam in hurricane-hit Puerto Rico have been evacuated.
The evacuation was sparked after a crack was found in the Guajataca Dam thought to have been caused by Hurricane Maria that swept through the island on Wednesday.
Nearly 40 centimeters of rain fell on the mountains surrounding the Guajataca Dam in northwest Puerto Rico swelling the reservoir behind the dam.
Some 70,000 people were evacuated by bus as authorities tried to reach residents with warnings on Twitter. Much of the island remains without electricity after Maria knocked out power supplies.
An engineer inspecting the dam reported a "contained breach" that officials identified as a crack that could be the first sign of total failure of the dam, said Anthony Reynes, a meteorologist with the U.S. National Weather Service.
"There's no clue as to how long or how this can evolve. That is why the authorities are moving so fast because they also have the challenges of all the debris. It is a really, really dire situation," Reynes said.
Across the US territory, residents feared power could be out for weeks - or even months.
Some of the island's 3.4 million residents planned to head to the US to temporarily escape the desolation.
In San Juan, Neida Febus, 64, wandered around her neighbourhood with bowls of cooked rice, ground meat and avocado, offering food to anyone in need.
The damage was so extensive, she said, that she did not think the power would be turned back on until Christmas.
"This storm crushed us from one end of the island to the other," she said.
The death toll in Puerto Rico stood at six but is likely to rise.
The Federal Emergency Management Agency said three to four military planes would be flying to the island every day carrying water, food, generators and temporary shelters.
Puerto Rico governor Ricardo Rossello said his administration was trying to open ports so they island could receive food, water, generators, camp beds and other supplies.
The entire island has been declared a federal disaster zone.