Dominica's prime minister says 20 people have died and the island has been set back 20 years in the damage inflicted by Tropical Storm Erika.

Roosevelt Skerrit said in a televised address late Friday that hundreds of homes, bridges and roads have been destroyed.

He said authorities still have not been able to reach some communities cut off by landslides, mudslides and flooding.

Tropical Storm Erika dumped 15 inches of rain on the mountainous island and unleashed destructive flooding.

Police in the lush and mountainous island of Dominica expected to reach isolated communities via the ocean because of impassable roads and bridges. The Barbados-based Caribbean Disaster and Emergency Management Agency also pledged assistance. Ronald Jackson, the agency's executive director, said in a phone interview that at least two helicopters would arrive early Friday in Dominica carrying supplies and two medics from Trinidad.

"The only way into Dominica at this time is via helicopter," he said.

Erika downed trees and power lines in Dominica as it unleashed heavy floods that swept cars down streets and ripped scaffolding off some buildings