The Dominican Republic is on alert as the first Atlantic hurricane of the 2011 season approaches the Caribbean nation after passing over the U.S. territory of Puerto Rico.

Forecasters say Hurricane Irene is expected to hit the northern coast of the Dominican Republic on Monday, with maximum winds of 120 kilometers an hour. A tropical storm warning also is in effect for Haiti, which neighbors the Dominican Republic on the western part of Hispaniola island.

Hundreds of thousands of Haitians remain in makeshift shelters after a devastating earthquake last year.

Irene intensified into a hurricane over Puerto Rico before dawn Monday, flooding the streets, knocking down trees and cutting power to about 1 million residents. There were no immediate reports of casualties.

The U.S. National Hurricane Center in Miami says Irene is likely to approach the southeastern U.S. state of Florida later in the week. Authorities there began preparations on Monday for the storm's arrival.