Russian intelligence agencies have mounted a campaign to undermine confidence in Pfizer Inc.’s and other Western vaccines, using online publications that in recent months have questioned the vaccines’ development and safety, U.S. officials said.

An official with the State Department’s Global Engagement Center, which monitors foreign disinformation efforts, identified four publications that he said have served as fronts for Russian intelligence.

The websites played up the vaccines’ risk of side effects, questioned their efficacy, and said the U.S. had rushed the Pfizer vaccine through the approval process, among other false or misleading claims.

Though the outlets’ readership is small, U.S. officials say they inject false narratives that can be amplified by other Russian and international media.

The Sputnik V vaccine being administered at a site in Saint Petersburg, Russia, last month.

“We can say these outlets are directly linked to Russian intelligence services,” the Global Engagement Center official said of the sites behind the disinformation campaign. “They’re all foreign-owned, based outside of the United States. They vary a lot in their reach, their tone, their audience, but they’re all part of the Russian propaganda and disinformation ecosystem.”

In addition, Russian state media and Russian government Twitter accounts have made overt efforts to raise concerns about the cost and safety of the Pfizer vaccine in what experts outside the U.S. government say is an effort to promote the sale of Russia’s rival Sputnik V vaccine.

“The emphasis on denigrating Pfizer is likely due to its status as the first vaccine besides Sputnik V to see mass use, resulting in a greater potential threat to Sputnik’s market dominance,” says a forthcoming report by the Alliance for Securing Democracy, a nongovernmental organization that focuses on the danger that authoritarian governments pose to democracies and that is part of the German Marshall Fund, a U.S. think tank.

The foreign efforts to sow doubts about the vaccine exploit deep-seated anxieties about the efficacy and side effects of vaccines that were already prevalent in some communities in the U.S. and internationally. Concern about side effects is a major reason for vaccine hesitancy, according to U.S. Census Bureau data made public last month.

Kremlin spokesman Dmitry Peskov denied that Russian intelligence agencies were orchestrating articles against Western vaccines and said U.S. officials were mischaracterizing the broad international debate over vaccines as a Russian plot.