* Barbuda island suffers "total carnage" - prime minister
* About 70 percent Puerto Rico without power
* Storm likely to hit Florida Saturday or Sunday
* Two other hurricanes formed on Wednesday (Raises death toll, adds details throughout)
SAN JUAN, Puerto Rico, Sept 7 (Reuters) - Hurricane Irma plowed past the Dominican Republic on Thursday after devastating a string of Caribbean islands and killing at least 11 people as one of the most powerful Atlantic storms in a century took aim at Florida.
With winds of around 180 mph (290 kph), the storm lashed several small islands in the northeast Caribbean, including Barbuda, St. Martin and the British Virgin Islands, tearing down trees, flattening homes and causing widespread damage.
The eye of the hurricane passed north of Puerto Rico early Thursday, battering the U.S. territory with high winds and heavy rains and leaving nearly 70 percent of the population without electricity, Governor Ricardo Rossello said.
The eye of Irma was moving west-northwest off the northern coast of the Dominican Republic on Thursday morning, the National Hurricane Center (NHC) said.
Irma's precise course remained uncertain but it was likely to be downgraded to a Category 4 storm by the time it makes landfall in Florida, according to the NHC.
Irma has become a little less organized over the past few hours but the threat of a direct hurricane impact in Florida over the weekend and early next week was increasing, the NHC said.
Authorities in the Dominican Republic ordered the evacuation of towns along the northern Atlantic coast, as the storm ground toward the port and tourist destination of Puerto Plata.
"There is a lot of wind and rain," Puerto Plata Assistant District Attorney Juan Carlos Castro Hernandez told Reuters by telephone. "We expect things to get worse."
At least 8 people were killed in the tiny French-Dutch island of Saint Martin, with 23 others injured, and the toll was likely to rise as emergency services reached isolated communities, officials said.
"It is an enormous disaster. Ninety-five percent of the island is destroyed. I am in shock," Daniel Gibbs, chairman of a local council on Saint Martin, told Radio Caribbean International.
Television footage from the island showed a damaged marina with boats tossed into piles, submerged streets and flooded homes. French President Emmanuel Macron spoke with British Prime Minister Theresa May on Thursday to coordinate an emergency humanitarian response.
Amid criticism from many residents that the British government could have done more to help its territories, Foreign Office Minister Alan Duncan said a Royal Navy ship would reach the affected islands on Thursday with tents, vehicles and other relief equipment.
"Anguilla received the hurricane's full blast. The initial assessment is that the damage has been severe and in places critical," he told parliament.
One person was killed on the island and roads were blocked, with damage to the hospital and airport, power and phone service, Anguilla emergency service officials said.
In Puerto Rico, Rossello was due to fly to the worst effected areas of the island on Thursday but said it was to early to estimate the cost of the damage. The streets of the capital San Juan were littered with downed tree limbs and signs, with many street lights out.
Juan Pablo Aleman, a restaurant owner, said he had ridden out the storm in his 11th-floor apartment.
"The building moved, shook a few times. A lot of shingles came off and some windows broke," he told Reuters. "If it had gone a little more to the south, it would have been catastrophic."
Irma was the strongest hurricane ever recorded in the Atlantic Ocean and one of the five most forceful storms to hit the Atlantic basin in 82 years, according to the NHC.
The hurricane was on track to reach Florida on Saturday or Sunday, and would become the second major hurricane to hit the U.S. mainland in as many weeks after Hurricane Harvey.
Florida Governor Rick Scott said it was unclear whether Irma would hit the state's east or west coast but told residents to beware of the surge from the powerful winds.
"The storm surge can kill you," Scott said on the "CBS This Morning" program on Thursday. He urged people to heed local officials and be ready when the call came to leave their area, promising the government would provide transportation to those who need it.
"YOU'RE ON YOUR OWN"
Florida emergency management officials began evacuations in advance of Irma's arrival, ordering tourists to leave the Florida Keys.
Roman Gastesi, the administrator of Monroe County, which encompasses the Florida Keys, told CNN that streets were empty in Key West and 90 percent of businesses were closed. County officials, including police and emergency workers, would be leaving, he said.
"If you're going to stay, you're on your own," Gastesi said.
U.S. President Donald Trump was monitoring Irma's progress. The president owns the waterfront Mar-a-Lago estate in Palm Beach, Florida.
Trump approved emergency declarations for the state, Puerto Rico and the U.S. Virgin Islands, mobilizing federal disaster relief efforts.
The island of Barbuda, one of the first hit by the storm, was a scene of "total carnage," said Gaston Browne, prime minister of Antigua and Barbuda, adding that the tiny two-island nation will seek international assistance.
Browne told the BBC that about half of Barbuda's population of some 1,800 were homeless while nine out of 10 buildings had suffered some damage and many were destroyed.
"We flew into Barbuda only to see total carnage. It was easily one of the most emotionally painful experiences that I have had," Browne said in an interview on BBC Radio Four, adding that it would take months or years to restore some level of normalcy to the island.
Browne said one person was killed on Barbuda. A surfer was also reported killed on Barbados.
Hurricane warnings were in effect for the Bahamas and the British overseas territory of the Turks and Caicos, the NHC said.
In Cuba, 90 miles (145 km) south of the Florida Keys, a hurricane alert was issued for the central and eastern regions, as residents in Havana waited in lines to stock up on food, water and gasoline.
Two other hurricanes formed on Wednesday.
Katia in the Gulf of Mexico posed no threat to the United States, according to U.S. forecasters. Hurricane Jose was about 815 miles (1,310 km) east of the Caribbean's Lesser Antilles islands, and could eventually threaten the U.S. mainland.
The storm activity comes after Harvey claimed about 60 lives and caused property damage estimated to be as much as $180 billion in Texas and Louisiana.
(Reporting by Jorge Pinedo in Santo Domingo, Makini Brice in Port au Prince, Susan Heavey and Ian Simpson in Washington, Brendan O'Brien in Milwaukee, Estelle Shirbon in London, Matthias Blamont and Jean-Baptiste Vey in Paris, Toby Sterling in Amsterdam; Writing by Daniel Flynn; Editing by Matthew Mpoke Bigg and Jeffrey Benkoe)