Photo: The Canadian Press. All rights reserved.

This NOAA satellite image taken Friday, Aug. 1, 2014, at 1:45 p.m. EDT shows a mass of clouds associated with Tropical Storm Bertha near the Lesser Antilles with most of the clouds and rain displayed to the east of the storm centre. (AP Photo/Weather Underground)

Tropical Storm Bertha swirled through the eastern Caribbean on Friday as it unleashed heavy rains and strong winds in the region.

The storm's maximum sustained winds increased slightly Friday to 50 mph (85 kph), but no significant change in strength is expected in the next two days. Bertha was centred about 20 miles (30 kilometres) northwest of Martinique and was moving west-northwest at 24 mph (39 kph).

Tropical storm warnings were issued for Dominica, Martinique, the U.S. and British Virgin Islands and Puerto Rico. A tropical storm watch is in effect for the eastern Dominican Republic, the southeastern Bahamas and the Turks and Caicos islands.

Bertha passed just north of the French Caribbean island of Martinique, where officials said they were bracing for heavy winds and dangerous swells and urged everyone to stay indoors.

Government spokeswoman Audrey Hamann said in a phone interview that power losses were reported in several parts of the island and that authorities were trying to restore electricity. She said officials did not immediately know how many people were affected.

In St. Lucia, emergency management officials reported overcast skies with constant showers.

"It looks like we're just beginning to see the start of it," Junius St. Hill, acting lead fireman, said by phone. "Today's a holiday, so most people would be indoors anyway."

In Dominica, Prime Minister Roosevelt Skerrit ordered all businesses closed by Friday afternoon to prepare for the storm.

Antigua-based regional airline LIAT also cancelled several flights in Dominica and St. Lucia.

The storm generated some rain and wind as it passed just north of Barbados earlier Friday, but no damage has been reported, said Judy Thomas, director of the island's emergency management agency.

"It's had no impact," she said in a phone interview.

Bertha is expected to generate up to 3 inches (8 centimetres) of rain across the eastern and northern Caribbean, with isolated amounts of up to 6 inches (15 centimetres) in certain areas.

Officials in Puerto Rico are welcoming the rainfall amid a moderate drought that has hit the island's southern region and a small portion in the northeast. More than half of the U.S. territory also is experiencing abnormally dry conditions, with the government reporting $20 million in crop losses.