(CNN) -- Hurricane Igor ballooned into a major Atlantic Ocean hurricane on Sunday with top winds of 140 mph (220 kph), but remained far from land, forecasters reported.
Igor grew from a tropical storm to a hurricane Saturday night and swelled from a Category 2 to a Category 4 storm in the three hours between the late morning and early afternoon advisories from the National Hurricane Center. Forecasters described it as "large and powerful."
As of 5 p.m. ET, the storm was about 1,065 miles (1,715 kilometers) east of the northern Leeward Islands and was moving west at about 14 mph (22 kph), the center reported.
Forecasters had expected the storm to strengthen, but had predicted it would not become a Category 3 storm until Monday. The storm could still gain some strength, according to the hurricane center.
Hurricane-force winds extended up to 40 miles (65 km) from Igor's center, and tropical storm force winds can be felt up to 160 miles (260 km) from the center, according to the agency. As of Sunday morning, no coastal watches or warnings were in effect, and the storm is expected to turn to the west-northwest in the next two days.
Igor's eye is 20 nautical miles across, said CNN meteorologist Jacqui Jeras. While the storm is expected to stay north of the Caribbean islands, the islands could experience some wind and rain because of Igor's size, she said.
Meanwhile, in the far eastern Atlantic, Tropical Depression 12 formed Sunday morning. As of 5 p.m. ET, its center was about 150 miles (240 kilometers) southeast of the southernmost Cape Verde Islands
The depression was spreading rain over the Cape Verde Islands Sunday, according to the hurricane center. Tropical storm conditions were expected to reach the islands Sunday night.
The depression's maximum sustained winds were at 35 mph (55 kph). It was moving west at 14 mph (22 kph). "On the forecast track, the center of the depression is expected to pass near the southernmost Cape Verde Islands tonight and Monday," forecasters said.
Some strengthening is expected, and the storm could become Tropical Storm Julia Sunday night or Monday.
A tropical storm warning was issued for the southern islands, including Maio, Sao Tiago, Fogo and Brava. A tropical storm warning means that tropical storm conditions are expected in the warning area. The hurricane center says such conditions could be expected in the region within the next 12 to 24 hours.
Tracking maps show the depression moving into the open Atlantic, becoming a Category 1 hurricane next week.