MIAMI, Florida (CNN) -- Hurricane Bertha, a Category 1 storm, was causing large swells and high surf on Bermuda beaches Friday morning as it crept toward the Atlantic island.

Forecasters could not predict how close Bertha's strong winds would come to Bermuda, but a tropical storm watch may be issued there Friday, the National Hurricane Center said.

As of 5 a.m. Friday, Bertha's center was about 350 miles (565 kilometers) south-southeast of Bermuda, moving northwest at near 7 mph (11 kph). The storm's maximum sustained winds had decreased slightly to about 85 mph (140 kph) with higher gusts, forecasters said.

The storm is expected to slow down its forward progress and turn toward the north and northeast in the next two days, the hurricane center said.