Tropical storm warnings were posted for Puerto Rico, the Virgin Islands, St. Lucia, Dominica, Martinique, Culebra and Vieques, meaning that the storm could hit within 24 hours. The National Hurricane Center in Miami said parts of Puerto Rico could be doused with up to 10 inches of rain.
In the Dominican Republic, the government posted a tropical storm watch for the eastern part of the country.
At 11 a.m. AST, Bertha's center was about 150 miles east-southeast of Martinique and 70 miles northeast of Barbados. The storm was moving north-northwest at a speedy 21 mph.
A U.S. Air Force hurricane hunter plane reported Bertha to be "slightly stronger but disorganized."
The storm's maximum sustained winds were about 50 mph, with tropical storm winds extending as much as 115 miles from the center.
Forecasters said Bertha was expected to continue on its current course for the next couple of days without getting any stronger.