Published: Sep 12, 2010 7:48 PM EDT
Updated: Sep 12, 2010 4:48 PM EDT

MIAMI (AP) - Forecasters say Hurricane Igor has rapidly strengthened to a Category 4 storm in the open Atlantic. It doesn't immediately threaten land.

Meanwhile, a tropical depression off the coast of Africa prompted officials to issue a tropical storm warning for parts of the Cape Verde Islands on Sunday.

Igor had maximum sustained winds of 135 mph (215 kph) and was moving west at 14 mph (22 kph). Some additional strengthening is expected in the next two days.

The center of the storm was located about 1,120 miles (1,800 km) east of the Northern Leeward Islands.
      
Forecasters also warned that the newly formed depression could strengthen into a tropical storm as early as Sunday night. The storm warning was issued for the southern Cape Verde Islands, including Maio, Sao Tiago, Fogo and Brava.