Tropical storm Dorian made a last-minute shift in its path on Tuesday, threatening Puerto Rico with a direct hit as forecasters said it could reach near-hurricane strength in its approach to the U.S. territory.

The storm is expected to pass over or near western and central Puerto Rico on Wednesday as authorities warned of landslides, widespread flooding and power outages.

"Practically the entire island will be under sustained tropical storm force winds," said Roberto Garcia, director of U.S. National Weather Service San Juan, during a news conference late Tuesday.

However, he said the forecast could change overnight, adding that late shifts occur with storms such as Dorian that do not have a well-defined centre.

Dorian was located about 480 kilometres southeast of Ponce, Puerto Rico, on Tuesday night. The U.S. National Hurricane Center (NHC) said it had maximum sustained winds of 85 km/h and was forecast to strengthen during the next 24 hours as it moves west-northwest at 20 km/h. The storm is expected to dump 10 to 15 centimetres of rain with isolated amounts of 20 centimetres.

The change in the storm's course concerned many across the U.S. territory, where some 30,000 homes still have blue tarps as roofs nearly two years after Hurricane Maria. The island's 3.2 million inhabitants still depend on a shaky power grid that has remained prone to outages since it was destroyed by the Category 4 storm.

A man fixes the tin roof of a stilt house in Puerto Rico in preparation for the storm. (Ricardo Arduengo/Reuters)

The NHC issued a hurricane watch for Puerto Rico and a tropical storm warning for Puerto Rico, Vieques, Culebra and the U.S. Virgin Islands. A tropical storm watch was in force for the Dominican Republic from Samana to Puerto Plata.

In anticipation of the storm, U.S. President Donald Trump declared a state of emergency late Tuesday for Puerto Rico, the White House said in a statement. The declaration allows for the Federal Emergency Management Agency to provide assistance in coordination with disaster preparedness efforts on the island.

Dorian has already caused power outages and downed trees in Barbados and St. Lucia, and a still-uncertain long-term track showed the storm near Florida over the weekend.

In Puerto Rico, some grocery stores ran out of bottled water as people rushed to buy supplies including generators and filled their cars with gasoline.

"The biggest problem will be the rain," said Roberto Garcia, a forecaster with the National Meteorological Service in Puerto Rico.

Puerto Rico Gov. Wanda Vazquez signed an executive order Monday declaring a state of emergency and provided a list of all the new equipment that public agencies have bought since Hurricane Maria.

"I want everyone to feel calm," she said. "Agency directors have prepared for the last two years. The experience of Maria has been a great lesson for everyone."

Officials also said public schools and government offices would remain closed through at least Thursday.

"We learned our lesson quite well after Maria," Vazquez said. "We are going to be much better prepared."

Volunteer members of the Roving Response Team remove a tree blocking a road Tuesday after tropical storm Dorian passed overnight in Brighton St. George, Barbados. (Nigel R Browne/Reuters)

Dorian was expected to move near the Turks and Caicos Islands and southeastern Bahamas on Thursday night or Friday.

Meanwhile, a new tropical depression formed Monday between the U.S. eastern coast and Bermuda. It was located about 510 kilometres southeast of Cape Hatteras in North Carolina and was moving north at 11 km/h Tuesday with maximum sustained winds of 55 km/h.

It was expected to become a tropical storm on Wednesday and continue blowing off the U.S. East Coast this week on a path to Canada's Atlantic provinces.