(Reuters) - U.S. regulators on Thursday kicked off a process that could result in mandatory disclaimers on election ads that appear on social media, a reaction to anonymous, Russia-linked ads on sites such as Facebook ahead of the 2016 U.S. elections.

A 3D-printed Facebook logo is displayed in front of the Russian flag, in this illustration taken October 25, 2017. REUTERS/Dado Ruvic/Illustration

The Federal Election Commission voted 5-0 to begin drafting rules that would impose disclaimers. The commission previously declined to require such labels after Facebook Inc and Alphabet Inc’s Google complained they would be impractical.

“Disclaimers on paid digital and internet-based advertisements are one tool identified as a mechanism for exposing foreign-paid advertisements,” three Republican members of the commission said in a written motion.

Facebook, Google and Twitter Inc have been at the center of a U.S. political firestorm after allegations that people in Russian used their services to spread divisive political messages in the United States ahead of and after last year’s presidential election.

Three U.S. congressional committees have held hearings, pressuring the companies to explain why they were not able to detect the alleged election interference and what Silicon Valley would do to prevent a recurrence.

The Russian government has denied trying to meddle in the election, in which Republican Donald Trump defeated Democrat Hillary Clinton.

Facebook, Google and Twitter have responded by proposing self-regulation of advertising, while also expressing a willingness to work with U.S. lawmakers and saying they would like the election commission to provide clarity on when disclaimers are required.

Facebook, the world’s largest social network, told the commission in written comments that it “strongly supports” the commission “providing further guidance to committees and other advertisers regarding their disclaimer obligations.”

Facebook Chief Operating Officer Sheryl Sandberg said on Wednesday that Facebook had begun testing a feature letting people see the ads any advertiser is running, including ads not targeted at them directly. The test is occurring in Canada and was expected to roll out in the United States next year, she said in a Facebook post.

About 126 million Americans may have seen 80,000 Russia-linked posts on Facebook over a two-year period, the company has said.

Thursday’s commission vote was bipartisan. It has three Republican members, one Democrat and one independent, as well as one vacant seat.

The vote “should serve as a signal to everyone in government whose job it is to protect our democracy that failure to act is no longer an option,” Democratic Commissioner Ellen Weintraub said in a statement.