By Parisa Hafezi, Yomna Ehab and Humeyra Pamuk DUBAI/WASHINGTON, June 15 (Reuters) - U.S. and Iranian officials said they had reached an agreement to end their war and reopen the Strait of Hormuz, a preliminary pact that sent oil prices falling but leaves the fate of Tehran's nuclear program to further negotiations. While still a framework, the deal marked the biggest breakthrough towards resolving the conflict that has killed thousands and upended energy markets since it began with joint U.S.-Israeli strikes on Iran in February.