The National Hurricane Center in Miami says the eye wall of Category 5 Maria is moving onshore over Dominica.
Forecasters say that the "potentially catastrophic" hurricane with 260 kph winds will likely travel over the Leeward Islands and the extreme northeast Caribbean through Monday night and Tuesday.
They expect Hurricane Maria will approach Puerto Rico and the Virgin Islands on Tuesday night and Wednesday.
Maria could dump 25 to 38 centimeters of rain over the Leeward Islands and the US and British Virgin Islands, causing flash floods and mudslides.
The National Hurricane Center in Miami said today that Air Force Reserve hurricane hunter planes found that Maria had strengthened into a storm with 260 km/h winds.
A series of Facebook posts by Dominica Prime Minister Roosevelt Skerrit is capturing the power of Hurricane Maria as it roars past the island as a Category 5 storm.
About 8:45 p.m. Skerrit writes: "The winds are merciless! We shall survive by the grace of God!"
A few minutes later, he writes that he can hear the sound of galvanised steel roofs coming off houses on the small, rugged island in the eastern Caribbean.
He then writes that he believes his home has been damaged. And three words: "Rough! Rough! Rough!"
A half-hour later he says "My roof is gone. I am at the complete mercy of the hurricane. House is flooding." Seven minutes later he posts that he has been rescued.
Maria was passing just south of Dominica late Monday and into early Tuesday on a path taking it toward Puerto Rico.
The storm was on a path that would take it near many of the islands already wrecked by Hurricane Irma and then on toward Puerto Rico and the Dominican Republic.
Maria could hit Puerto Rico on Thursday according to Ernesto Morales with the US National Weather Service in San Juan.
"This storm promises to be catastrophic for our island," he said. "All of Puerto Rico will experience hurricane force winds."
Overnight Maria was centred about 150 kilometres east-southeast of Dominica - or 95 kilometres east of Martinique - and heading west-northwest.
Hurricane warnings were posted for the US and British Virgin Islands, Guadeloupe, Dominica, St. Kitts, Nevis, Montserrat, Martinique and St. Lucia.
A tropical storm warning was issued for Antigua and Barbuda, Saba, St. Eustatius, St. Maarten and Anguilla.