The U.S. National Hurricane Center says Tropical Storm Irene is strengthening as it approaches Puerto Rico with winds of 110 kilometers per hour, nearing hurricane strength.

Forecasters say hurricane conditions are expected in Puerto Rico within hours. The Dominican Republic is also under a hurricane warning with the high winds, severe rain and high tides expected Monday.

A hurricane watch is in effect for the U.S. Virgin Islands and 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.