Puerto Rico grappled with the prospect of more flooding on top of an island-wide power outage following Hurricane Maria as the death toll from the powerful storm hit 21, most of them on the tiny Caribbean island of Dominica.

President Donald Trump declared Puerto Rico a disaster zone and said Maria had "obliterated" the US territory of 3.4 million people.

Hurricane Maria devastated Puerto Rico on Wednesday and left the entire island without power, bringing deadly winds and pounding rain that sent thousands scurrying to shelters.

Hurricane Maria closed in on the US Virgin Islands late Tuesday as the US National Hurricane Center warned of a "potentially catastrophic" impact from a storm that has already killed at least two people in the Caribbean.

Hurricane Maria strengthened into a "potentially catastrophic" Category Five storm as it barrelled into eastern Caribbean islands still reeling from Irma, forcing residents to evacuate in powerful winds and lashing rain.

After a succession of mixed messages on the U.S. stance on climate change, Secretary of State Rex Tillerson said overnight that the Trump administration was seeking "ways in which we can work with partners in the Paris climate accord."

"We want to be productive, we want to be helpful," Tillerson said on the CBS program "Face the Nation."

Hurricane Maria barrelled towards the storm-battered eastern Caribbean and was expected to strengthen on Monday as it churned along a path similar to that of megastorm Irma earlier in the month. 

The new storm, which the US National Hurricane Center warned could become a "major hurricane", threatens the French territory of Guadeloupe, which was the staging area for relief operations for several islands hit by Irma.

The White House has pushed back at a European suggestion it was softening its stance on the Paris climate accord, insisting Washington will withdraw from the agreement unless it can re-enter on more favorable terms.

Shaken residents in central Vietnam were piecing their homes and businesses back together Saturday after a powerful typhoon hammered large swathes of coast and killed at least four people.  

Some 30 environment ministers will push forward on the Paris climate accord at a meeting Saturday requested by Canada, China and the European Union.

Heavy rain and wind lashed Vietnam's central coast Friday as Typhoon Doksuri made landfall, prompting mass evacuations as officials predicted it could be the most powerful storm in a decade.