Hurricane Irma has strengthened into an "extremely dangerous" Category 5 storm, the National Hurricane Center said, threatening to slam into northeastern Caribbean islands and Puerto Rico by Wednesday before possibly heading for the US mainland of Florida .
Irma was churning Tuesday morning in the Atlantic about 270 miles east of Antigua and Barbuda, heading west with maximum sustained winds of 175 mph -- well above the 157 mph threshold for a Category 5 -- the hurricane center said.