MIAMI — Tropical Storm Gabrielle strengthened into a hurricane Sunday afternoon southeast of Bermuda in the Atlantic Ocean, forecasters said.

The Miami-based National Hurricane Center said Gabrielle’s maximum sustained winds increased to 75 mph (120 kph), up 10 mph (16 kph) from hours earlier to become a Category 1 hurricane. The storm was centered some 320 miles (515 kilometers) southeast of Bermuda and moving to the north-northwest at 10 mph (17 kph).

The hurricane center said in an afternoon advisory that Gabrielle could become a major hurricane in the early part of this week as it is expected to undergo steady to rapid intensification over the next day or so.

On the current forecast track, Gabrielle was expected to pass east of Bermuda on Monday.

A hurricane hunter aircraft found the storm at hurricane strength and moving on a more north-northwest track. But the center said a more northerly course was expected on Monday.

No coastal watches or warnings are in effect, but meteorologists urged interests in Bermuda to keep a close watch on the storm’s progress.

Hurricane-force winds extend outward up to 25 miles (40.2 kilometers) from the core of the storm, while tropical-storm force winds extend outward up to 175 miles (280 kilometers), the center said.

Large ocean swells kicked up by Gabrielle are impacting Bermuda and are expected to reach the Eastern Seaboard from North Carolina northward into Atlantic Canada over the coming days.