(Reuters) – September 4, 2008 -- Hurricane Ike strengthened rapidly into an fiercely dangerous Category 4 hurricane in the open Atlantic on Wednesday and Tropical Storm Hanna intensified to a lesser degree as it swirled over the Bahamas toward the southeast U.S. Coast.

  Ike posed no immediate threat to land but strengthened explosively, growing in the space of a few hours from a tropical storm to an intense Category 4 hurricane on the five-step Saffir-Simpson intensity scale.

  Ike had top sustained winds near 145 mph (230 kph) as it swept across the open waters of the west-central Atlantic 550 miles (885 km) northeast of the Leeward Islands, the U.S. National Hurricane Center said. It was moving west-northwest near 17 mph (28 kph).

  It was forecast to head for the southern Bahamas early next week but it was too early to tell whether it would threaten land, the forecasters said.

  It was also too soon to say whether Ike would threaten U.S. oil and natural gas producers in the Gulf of Mexico.

  The hurricane center's Web site, with updates and graphics, is at http://www.nhc.noaa.gov/index.shtml.

  Hanna's torrential rains had already submerged parts of Haiti, stranding residents on rooftops and prompting President Rene Preval to warn of an "extraordinary catastrophe" to rival a storm that killed more than 3,000 people in the flood-prone Caribbean country four years ago.

  Hanna was forecast to move over the central and northern Bahamas on Thursday, strengthening back into a hurricane with winds of at least 74 mph (119 kph) before hitting the U.S. coast near the North Carolina-Virginia border on Saturday.

  The government of the Bahamas had ended a hurricane warning for the northwestern part of the islands, meaning a tropical storm warning was now in effect for all of the Bahamas and for the Turks and Caicos Islands, the hurricane center said.