Maria is moving roughly along the same path as Hurricane Irma, which devastated the region this month.
Hurricane warnings have been issued for Guadeloupe, Dominica, St Kitts and Nevis, Montserrat, Martinique, St Lucia and the US and British Virgin Islands, and a hurricane watch is in effect for Puerto Rico, St Martin, St Barts, Saba, St Eustatius and Anguilla.
Some of these islands are still recovering after being hit by Irma - a category five hurricane which left at least 37 people dead and caused billions of dollars' worth of damage.