China has been buying up ads on U.S. social-media sites and adopting online tactics reminiscent of Russian disinformation campaigns in an apparent attempt to shape the story internationally about the coronavirus response, according to researchers analyzing the activity.

The efforts include ad purchases on Facebook Inc. promoting the English-language arms of Chinese state-media outlets, as well as posts there and on Twitter Inc.’s platform that in some cases disparage U.S. efforts to fight the global pandemic, the researchers...