MIAMI, Florida (CNN) -- Hurricane Bertha strengthened to a Category 3 storm Monday afternoon, but slowed in pace hours after becoming the first hurricane of the 2008 Atlantic season, according to the National Hurricane Center.
A satellite picture from 4:45 p.m. ET Monday shows Hurricane Bertha over the Atlantic.
At 5 p.m., Bertha was 730 miles (1,175 kilometers) east of the northern Leeward Islands and was moving west-northwest near 12 mph -- down from the morning's 15 mph.
Bermuda could be affected by the hurricane this weekend.
"It is still way too soon to determine whether or not Bertha will affect Bermuda," the center said.
There is a slight chance Bertha could make landfall in the United States.
"A gradual turn toward the northwest with a decrease in forward speed is expected over the next couple of days," the center said. See Bertha's projected path »
Its maximum sustained winds were clocked at 115 mph, up from 90 mph earlier in the day.
A Category 3 storm has wind speeds of 111 mph to 130 mph. Learn more about hurricanes »
The storm formed Thursday in the far eastern Atlantic, off the coast of Africa, near the southern Cape Verde Islands. It strengthened into a hurricane early Monday.
The first tropical storm of the season, Arthur, formed May 31 near the coast of Belize and dumped heavy rain on Central America and southern Mexico.
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Just hours after Tropical Storm Bertha became the first hurricane of the , Bertha strengthened to a "major" storm at category three on the five-step  scale of hurricane intensity, the  (NHC) said.

Five-day track map of Hurricane Bertha.At 5 p.m. EDT, the storm was packing sustained winds of 120 miles an hour and was located about 695 miles east-northeast of the Northern  and about 1085 miles southeast of Bermuda. Forecasters say that Bertha is moving west-northwest at about 12 miles an hour and is expected to slow down in the next couple of days. "A gradual turn toward the northwest with a decrease in forward speed is expected over the next couple of days," the NHC said.

While the NHC noted that it is still too early to tell if Bertha will strike land, residents are being urged to continue to monitor the storm's progress. Further strengthening is unlikely, according to the NHC, due to unfavorable atmospheric conditions.

Bertha first formed on Thursday in the eastern Atlantic off of Africa. The storm is the easternmost tropical storm to form over the Atlantic in the month of July.