MIAMI, July 18 (Reuters) - Tropical Storm Bertha strengthened on Friday as it accelerated toward the northeast over the open Atlantic in what was expected to be its final days as a storm, the U.S. National Hurricane Center said.

On its way to becoming one of the longest-lived tropical storms on record, Bertha was located around 725 miles (1,170 km) south of Cape Race, Newfoundland, by 11 a.m. EDT (1500 GMT) and moving to the northeast at a speedy 18 miles per hour (30 km per hour).

The top sustained winds of what was the first hurricane of the 2008 Atlantic storm season had increased to 65 mph (100 kph), but cooler waters in its path should prevent Bertha from strengthening further, the Miami-based hurricane center said.

Bertha, which raked Bermuda on Monday but caused no injuries in the British colony, was expected to start to fade as a tropical storm late on Saturday, by which point it would rank as one of the longest-lived Atlantic tropical storms since records began in 1851.

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.

Hurricane forecasters have predicted this season would be average or above-average.

An average season has around 10 storms, of which six strengthen into hurricanes with top sustained winds of at least 74 mph (119 kph). The record-busting 2005 season spawned 28 storms, including Hurricane Katrina that produced devastating flooding in New Orleans and elsewhere on the U.S. Gulf Coast.

The tropics on Friday were quite busy.

In addition to Bertha, the U.S. hurricane center was keeping an eye on an area of disturbed weather northwest of Aruba that could become a tropical depression -- the precursor to a tropical storm -- as it headed in a westerly direction toward Mexico's Yucatan Peninsula and the Gulf of Mexico.

Energy markets watch Atlantic storms carefully because of their potential to disrupt oil and gas drilling in the Gulf, where the United States gets a third of its domestically produced crude.

Another area of disturbed weather off the coast of Georgia also could become a tropical depression as it headed in a northerly or northeasterly direction, the hurricane center said. (Reporting by Michael Christie, Editing by Vicki Allen)

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