WASHINGTON—U.S. officials said that they are closely monitoring developments at or near a military complex outside Tehran, where Iranian officials said a fire killed two workers at an explosives plant.

Western officials have long been concerned that Iran’s Parchin military complex played a role in what they charge was the government’s effort to develop nuclear weapons. They suspect the site was used for explosives tests conducted as part of nuclear-weapons research, although Iran has denied that it is attempting to build nuclear...