The storm's maximum sustained winds early Friday are near 45 mph (75 kph) with no significant change in strength expected over the next two days.
In addition to Puerto Rico and the Virgin Islands, tropical storm warnings are in effect for Babados, St. Lucia, Dominica, Vieques and Culebra. A tropical storm watch is in effect for St. Vincent and the Grenadines.
Bertha is centered about 170 miles (270 kilometers) east of Barbados and is moving west-northwest near 20 mph (31 kph).
The U.S. National Hurricane Center says Bertha is expected to pass near or north of Barbados Friday afternoon.