The Trump administration unveiled new sanctions against Cuba and Venezuela, part of an effort to target what White House national-security adviser John Bolton called a “troika of tyranny” that includes Nicaragua.

Mr. Bolton on Thursday praised Latin American leaders such as Brazil’s newly elected conservative firebrand, Jair Bolsonaro, as “positive signs for the future of the region,” but said the three targeted countries are being singled out for political oppression and human-rights violations.