CARACAS, Venezuela—The U.S. government and Venezuela’s opposition are preparing humanitarian aid shipments for the crisis-stricken country, setting the stage for a showdown with authoritarian leader Nicolás Maduro as his military backers face rising international pressure.

Over the weekend, USAID administrator Mark Green posted images on Twitter of hundreds of boxes carrying food for malnourished Venezuelan children and bearing a U.S. flag being loaded on to pallets for delivery. Meanwhile, Mr. Maduro’s detractors began working...