The U.S. National Hurricane Center says Tropical Storm Irene has strengthened into a hurricane, the first of the 2011 Atlantic hurricane season.

Irene, with winds of 120 kilometers per hour, is expected to strengthen further in the next two days.

A hurricane warning is in effect for parts of Puerto Rico and the northern coast of the Dominican Republic.

A hurricane watch is in effect for some of the northern coast of Haiti, and for the Central Bahamas.

Tropical storm conditions are expected in much of the rest of the region.

Irene's substantial rainfall of up to 25 centimeters, and twice that in isolated areas, could cause life-threatening flash floods and mudslides, as well as large and dangerous waves.

The current path of the storm takes it just north of the coast of Cuba heading toward the southeastern U.S. state of Florida.