Media playback is unsupported on your device Media caption Heavy rainfall from Hurricane Maria put stress on the Guajataca Dam

The danger of flooding on a Puerto Rico river remains "due to the threat of a failure" of a dam in the wake of Hurricane Maria, officials say.

The National Weather Service (NWS) extended flash flood warnings for two areas downstream of Guajataca Dam.

All 70,000 residents in the areas under threat were initially told to flee but there are reports that the evacuation zone has since been narrowed.

At least 10 people have died since Maria ripped through Puerto Rico.

The hurricane - which is now expected to move into the open waters of the west Atlantic - also knocked out power to the whole island.

Puerto Rico Governor Ricardo Rossello tweeted (in Spanish) that he had assessed the damage to the dam by flying over the area.

Mr Rossello - who is gathering mayors from across the island for crisis talks - reiterated an earlier call from the authorities for local residents to leave their homes.

The Guajataca Dam, at the northern end of Lake Guajataca in the north-west, began to show signs of failing at 14:10 local time (18:10 GMT) on Friday, operators said.

The NWS on Saturday warned of flash flooding in Isabela and Quebradillas areas.

The agency has been urging residents to move to higher ground in an alert posted on its website.

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Mr Rossello earlier called Maria the worst hurricane in a century, warning that it could take months to restore electricity to all 3.4 million of the US island territory's residents.

Roofs were ripped off as strong winds battered Puerto Rico's capital city, San Juan.

Media playback is unsupported on your device Media caption Where has Hurricane Maria hit?

Maria, a category three storm with maximum sustained winds of nearly 185km/h (115mph), was about 1,030km south-south east from Cape Hatteras in North Carolina, the National Hurricane Center said in its latest update at 21:00 GMT on Saturday.

"On the forecast track, Maria should move away from the Bahamas into the open waters of the western Atlantic today," the NHC said.

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.