The Duke and Duchess of Sussex posted a joint statement on their Archewell Foundation website encouraging followers to unite and support organizations "doing critical work" to help those in need.
Prime Minister Ariel Henry declared a one-month state of emergency for the whole country and said he would not ask for international help until the extent of the damages was known.
The deaths come a day after Elsa caused widespread damage in several eastern Caribbean islands as a Category 1 hurricane, the first of the Atlantic season.