A tropical depression formed over the Turks and Caicos Islands on Saturday as it headed toward the Bahamas, dumping heavy rains on parts of Puerto Rico and the Dominican Republic, according to the U.S. Hurricane Center.

It is the fourth depression of the Atlantic hurricane season, and the system is expected to strengthen into a tropical storm on Sunday as it passes over or near the central Bahamas. Early Saturday evening, the depression had sustained winds of 55 km/h and was located about 130 kilometres northeast of Great Inagua island.

A tropical storm warning has been issued for the Turks and Caicos and for the southeast and central Bahamas, with forecasters saying it could bring up to 20 centimetres of rain. The depression was moving northwest near 18 km/h.

Earlier in the day, the storm downed several trees and power lines in Puerto Rico, leaving more than 17,000 people without power and nearly 5,600 without water.

Police said in a statement a small bridge collapsed Saturday in the central town of Barranquitas, isolating about 25 families in the area. No one was injured.