Tropical Storm Bertha approached the Caribbean on Friday as islands in the eastern region prepared for heavy rains and strong winds.

The storm's maximum sustained winds increased slightly Friday to 85 km/h but no significant change in strength is expected in the next two days. Bertha was centred about 70 kilometres east of Martinique and was moving west-northwest at 35 km/h.

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

"At this point in time, it's had no impact," she said in a phone interview, adding that the rain helped relieve a drought that began earlier this year.

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.

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.

Bertha is expected to generate up to eight centimetres of rain across the eastern and northern Caribbean, with isolated amounts of up to 15 centimetres in certain areas.

Puerto Ricans hope for rain

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 US in crop losses.

Strict rationing measures are scheduled to go into effect starting Aug. 6 if the storm doesn't generate enough rain.

"Whether it falls where it needs to fall, that's still to be seen," said Jose Antonio Estrada, National Weather Service meteorologist.

He said the storm is moving quickly and that its effects would be felt all day Saturday in Puerto Rico. Authorities closed El Yunque rainforest, a popular tourist attraction in northeast Puerto Rico, until further notice.

It rained less than an inch in June in Puerto Rico, compared with the month's average of more than four inches. July saw more rain, but the 8.64 centimetres that fell was still down from the average of 12 centimetres.

If dry conditions persist, hundreds of thousands of people living in and around the capital of San Juan would get water every other day.