Bertha's top sustained winds were around 60 miles per hour (95 km per hour) and the storm was moving to the east-southeast at 9 mph (15 kph), the Miami-based hurricane center said.

The storm was expected to turn directly east later on Thursday and then resume a course to the northeast that would eventually see it lose its tropical characteristics over the cooler waters of the North Atlantic.

Bertha has displayed an impressive resilience since forming on July 3, especially for a storm that formed so early in the six-month Atlantic hurricane season. The storm season begins on June 1 but rarely gets into gear before August.

"Bertha could become one of the top 10 longest-lived storms in history if it survives until the weekend," the hurricane center wrote in a discussion item on the storm.

Bertha brushed by the British colony of Bermuda on Monday, knocking out power to thousands of homes but causing no injuries.

It briefly grew into a "major" Category 3 hurricane on the five-step Saffir-Simpson scale, the same strength that Hurricane Katrina had attained when it came ashore near New Orleans in 2005.

Elsewhere in the tropics, an area of low pressure over the southern Caribbean had become less organized overnight and seemed less likely to become the third tropical depression of the summer, the hurricane center said.

That system had been of some concern to energy markets as computer models indicated it had the potential to head toward the oil and gas rigs of the Gulf of Mexico where the United States gets a third of its domestically produced crude.

Another area of disturbed weather near Central America was expected to bring potentially dangerous downpours to Honduras and Nicaragua but was not seen as a threat to the Gulf or the United States. (Reporting by Michael Christie, Editing by Philip Barbara)