FILE PHOTO - The Facebook logo is displayed on their website in an illustration photo February 1, 2017. REUTERS/Regis Duvignau/Illustration/File Photo

(Reuters) - Democratic U.S. lawmakers on Wednesday urged the Federal Election Commission to require transparency for social media advertising after Russians bought U.S. political ads on Facebook Inc FB.O in an alleged attempt to influence the 2016 elections.

Pressure is mounting on regulators and Silicon Valley companies to open up the opaque world of online political ads and to prevent governments from using them to sway elections or attempt other meddling.

“Social media platforms offer the ability to target millions of users based upon a wealth of highly detailed information,” the lawmakers wrote in a letter to the election commission.

“As we have seen, the low cost of reaching these users equips hostile foreign actors with a powerful new tool for disruption of our democratic process.”

Some 20 Democrats from the Senate and House of Representatives signed the letter.

Facebook, the world’s largest social network, said this month that an operation likely based in Russia spent $100,000 on thousands of Facebook ads promoting divisive messages before and after last year’s U.S. presidential election.

Russia has denied meddling in the U.S. election, in which Republican Donald Trump defeated Democrat Hillary Clinton. U.S. congressional committees and special counsel Robert Mueller are investigating the allegations.

The Federal Election Commission last week sought public comment on possible regulatory changes, and the commission is considering holding a public hearing, with Facebook and other tech companies as invited witnesses.

Facebook has said it is working with U.S. authorities who are investigating alleged Russian meddling and also taking steps, such as removing fake accounts, to prevent manipulation of its platform.