Some 70,000 people live downstream from the Guajataca Dam, at the northern end of Lake Guajataca in the north-west of the territory.
The overflow of Lake Guajataka in Puerto Rico after rain from Hurricane Maria leaves it in danger of failing. Photo: AFP
The compromised dam began to show signs of failing on Friday and people in its wake were under orders to evacuate, with the structure in danger of bursting at any time.
Puerto Rico's governor met mayors from around the ravaged island on Saturday after surveying damage to the dam.
"We saw directly the damage to the Guajataca dam," Governor Ricardo Rossello said in a Spanish-language Twitter message on Saturday while reinforcing his request that people leave the area as soon as possible.
"The fissure has become a significant rupture," Mr Rossello said separately at a news conference on Saturday.
The US National Weather Service said on its website the dam was still in danger of failing and triggering life-threatening flash floods.
"Stay away or be swept away," it warned.
Meanwhile, people across the island were struggling to dig out from the devastation left by the storm, which killed at least 25 people, including at least 10 in Puerto Rico, as it churned across the Caribbean.
In a development that could help the recovery effort, the Port of San Juan reopened, according to a Twitter message from the agency that operates it, allowing ships to unload supplies.
Severe flooding, structural damage to homes and virtually no electric power were three of the most pressing problems facing Puerto Ricans, said New York Governor Andrew Cuomo during a tour of the island.
"It's a terrible immediate situation that requires assistance from the federal government - not just financial assistance," said Mr Cuomo, whose state is home to millions of people of Puerto Rican descent.
"It is a dangerous situation today and it's going to be a long-term reconstruction issue for months," Mr Cuomo, a Democrat and potential 2020 presidential candidate, told CNN.
The aftermath of Hurricane Maria in Puerto Rico. Photo: AFP / FILE
Path of destruction
Maria, the second major hurricane to savage the Caribbean this month and the most powerful storm to strike Puerto Rico in nearly a century, carved a path of destruction on Wednesday. It knocked out electricity, apart from emergency generators, on the island of 3.4 million inhabitants.
Near the rain-swollen Guajataca River, in the northwest part of the island, floodwater littered with branches and debris engulfed the first floor of a number of homes and swamped vehicles that were left behind.
"We lost our house, it was completely flooded," said resident Carmen Gloria Lamb. "We lost everything, cars, clothes, everything."
Signs of the strain on Puerto Ricans were evident throughout San Juan, the capital.
Drivers had to wait up to seven hours at the few filling stations open on Saturday and lines of cars snaked for blocks. Hotels warned that guests might have to leave soon without fresh supplies of diesel to keep generators operating.
Water rationing also began on Saturday. Signs posted throughout San Juan's Old Town informed residents that service would return for two hours each day, between 5pm and 7pm, until further notice.
Telephone service was also unreliable, with many of the island's cell towers damaged or destroyed.
People swarmed under some of the towers, holding up their devices in the hopes of getting a signal.
The governor also extended a nightly curfew on Saturday, the Caribbean Business newspaper reported.
Debt Crisis
A woman pulls a travel case on a rock scattered road in the aftermath of Hurricane Irma in Fajardo, Puerto Rico. Photo: AFP
Maria struck Puerto Rico as a Category 4 storm on the five-step Saffir-Simpson scale as the island was already facing the largest municipal debt crisis in US history.
The storm may have caused an estimated $45 billion in damage and lost economic activity across the Caribbean, with at least $30bn of that in Puerto Rico, said Chuck Watson, a disaster modeler at Enki Research in Savannah, Georgia.
Elsewhere in the Caribbean, 14 deaths were reported on Dominica, an island nation of 71,000 inhabitants.
Two people were killed in the French territory of Guadeloupe and one in the US Virgin Islands. Two people died in the Dominican Republic on Thursday, according to media outlet El Jaya.
Maria still had sustained winds of up to 185km per hour on Saturday, making it a Category 3 hurricane, but was expected to weaken gradually over the next two days as it turned more sharply to the north.
Dangerous surf and rip currents driven by the storm were expected along the southeastern coast of the US mainland for several days, the National Hurricane Center said.
Maria hit about two weeks after Hurricane Irma, one of the most powerful Atlantic storms on record, killed more than 80 people in the Caribbean and the United States. It followed Hurricane Harvey, which also killed more than 80 people when it struck Texas in late August and caused flooding in Houston.
- Reuters / BBC