Hurricane Irene has churned into a stronger Category 2 storm and has cut a destructive path through the Caribbean as it heads toward the U.S. coast.

Forecasters at the U.S. National Hurricane Centre in Miami said Monday night that a hurricane hunter aircraft measured maximum sustained winds of 155 kph.

The storm raked Puerto Rico with strong winds and rain. It is spinning just north of the Dominican Republic on a track that could carry it to the U.S. Southeast as a major storm by the end of the week.

The first hurricane of the Atlantic season was a large system. Irene is forecast to grow into a Category 3 hurricane with winds of 184 kph over the Bahamas on Thursday.

It could be that strong as it heads to the U.S. coast. It could perhaps land in Florida, Georgia or South Carolina.

In Puerto Rico, Hurricane Irene cut power to more than a million people, downing trees and flooding streets, before heading out over warm ocean water.

There were no reports of deaths or major injuries, but Gov. Luis Fortuno declared a state of emergency and urged people to stay indoors to avoid downed power lines, flooded streets and other hazards.

"This isn't the time to go out to find out what happened ... This is the time stay in your homes," Fortuno said at a news conference.

Schools and many businesses in the U.S. island territory are closed as crews began assessing the damage. The governor said people should avoid going out because of the danger posed by fallen live power lines and other hazards. He says he doesn't yet know when power will be restored.

As a tropical storm, Irene downed trees and caused widespread power outages in the U.S. Virgin Islands as it churned kilometres from St. Croix, said Christine Lett, spokeswoman for the territory's emergency management agency.

si-storm-irene-300-66451f3f

Surfers jump off the rocks into the ocean as tropical storm Irene approaches to the island in Luquillo, Puerto Rico on Sunday. ((Ricardo Arduengo/Associated Press))

Forecasters earlier said Irene was likely to pass south of Puerto Rico, but the storm shifted north and was passing directly over the island, said Krizia Negron, a meteorologist with the U.S. National Weather Service in San Juan as the first 110 km/h winds began lashing the territory.

"The main threat from the winds is just starting and it's going to continue through the night," she said.

Puerto Rico's main airport was swamped with people, the usual Sunday crowds combined with people rushing to get off the island before the storm or stranded because flights to a number of other islands had been cancelled. There were long lines at check-in counters and at the airport hotel.

Strong winds and rain were battering the outlying Puerto Rican islands of Culebra and Vieques, where 150 tourists were evacuated, according to Gov. Luis Fortuno. At least 120,000 people were without power and another 13,000 without water as the storm approached.

Storm surge possible  

Authorities advised people to stay away from the ocean because Irene could bring a dangerous storm surge to the coast.

"I strongly recommend that swimmers and recreational boaters avoid the ocean and that the general public stay away from shoreline rocks until the tropical storm passes and weather and surf conditions normalize," said Capt. Drew Pearson, a U.S. Coast Guard commander.

In the U.S. Virgin Islands, Gov. John deJongh declared a state of emergency in order to impose storm curfews.  

"We've got what appears to be a direct hit on St. Croix," said governor spokesman Jean Greaux, referring to the largest and poorest of the U.S. Virgin Islands.

Emergency shelters were opened on St. Croix, where the port was closed. The Hovensa LLC refinery on St. Croix also closed its port because of the storm but operations remained normal at the refinery, one of the largest in the Western Hemisphere, said spokesman Steve Strahan.

Puerto Rico

The hurricane centre said the main impediment to the storm's progress over the next couple of days will be interaction with land.

If Irene passes over Hispaniola's mountains or over parts of eastern Cuba, the storm could weaken more than currently expected.

"However, if the system ends up moving to the north of both of those land masses it could strengthen more than expected," wrote forecaster Richard Pasch.

In the Dominican Republic, officials assured residents they had food available for 1.5 million people if needed. Also, soldiers and emergency management crews evacuated dozens of residents from high-risk areas along the southern coast.

"We have taken all precautions," presidential spokesman Rafael Nunez said.

Many stores in the capital of Santo Domingo closed Sunday even as people bought last-minute items like flashlights.

The hurricane centre's current forecast has Irene hitting southern Florida as a hurricane by Thursday.