'It could collapse tonight': Dramatic moment water gushes through crack in dam about to burst in Puerto Rico

This is the dramatic moment a dam failed in disaster-stricken Puerto Rico as thousands of residents scrambled to evacuate the river valley.

The footage, taken from a helicopter by a correspondent from Weather Nation TV, shows a deluge of water gushing through a crack in the infrastructure

Puerto Rico's emergency services have raced to evacuate tens of thousands of people as the damaged dam teetered on the brink of total collapse in the aftermath of Hurricane Maria.

As Puerto Ricans struggled without electricity after Maria wreaked devastation, killing at least 25 people across the Caribbean, officials urgently worked to clear the river valley in the island's northwest.

Christina Villalba, of the island's emergency management agency, said there was little doubt the dam was about to break.

"It could be tonight, it could be tomorrow, it could be in the next few days, but it's very likely it will be soon," she said, adding that authorities aimed to complete evacuations within hours.

Some 70,000 people live downstream from the earthen dam on the rain-swollen Guajataca River, said Governor Ricardo Rossello in a late-afternoon news conference on Friday.

Residents were being ferried to higher ground in buses, according to bulletins issued by the National Weather Service from its office in San Juan.

Mr Rossello said the rains sparked by Maria had cracked the dam and could cause fatal flooding. Puerto Rico's national guard has been drafted in to assist evacuations.

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.

The island remained entirely without electricity, except for emergency generators, two days later. Telephone service was also patchy.

Roofs were ripped from many homes and torrential downpours from the storm sent several rivers to record flood levels.

Officials confirmed on Friday at least six storm-related fatalities in Puerto Rico, an island of 3.4 million inhabitants. Earlier news reports had put the island's death toll as high as 15.

"We know of other potential fatalities through unofficial channels that we haven't been able to confirm," said Hector Pesquera, the government's secretary of public safety.

Maria struck Puerto Rico as a Category 4 storm as the island was already facing the largest municipal debt crisis in US history.

The storm was expected to have caused $45 billion in damage and lost economic activity across the Caribbean, with at least $30 billion of that in Puerto Rico, said Chuck Watson, a disaster modeller 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 when the storm roared past the Dominican Republic on Thursday, according to media outlet El Jaya.

Maria churned past Turks and Caicos Islands on Friday, then skirted away from the Bahamas, sparing both British territories from the brunt of the storm, according to the US National Hurricane Center.

It still had sustained winds of up to 125 miles per hour by late Friday, making it a Category 3 hurricane, but was expected to weaken gradually over the next two days.

Storm swells driven by Maria were expected to reach the southeastern coast of the US mainland on Friday, the NHC said.