WASHINGTON, Sept 7 (Reuters) - U.S. federal legislation may be needed to require social media companies to disclose more about how their platforms are used for political advertising, a senior Democratic lawmaker said on Thursday, amid new concerns about Russia's alleged efforts to influence the 2016 election.
Senator Mark Warner said Congress may need to update laws in order to make them consistent with rules governing television advertising, which is subject to higher disclosure requirements.
Facebook Inc said on Wednesday that an operation likely based in Russia had placed thousands of U.S. ads with polarizing views on topics such as immigration, race and gay rights on the social media site during a two-year-period through May 2017.
"An American can still figure out what the content is being used in TV advertising. You can go look at the ad," Warner said at a security conference.
"But in social media there is no such requirement. So, you know, we may need a legislative solution."
Facebook's disclosure renewed questions about the extent of foreign meddling in last year's U.S. presidential election.
U.S. intelligence agencies say there was a multi-pronged cyber-influence operation that included theft and leaks of Democratic emails, the spread of political hoaxes on social media and hacking attempts on state election systems, all ordered by Russian President Vladimir Putin to help Donald Trump, a Republican, defeat Democrat Hillary Clinton.
Russia has repeatedly denied the allegations.
On Thursday, liberal advocacy group Common Cause filed a complaint with the U.S. Justice Department and the Federal Election Commision alleging that unknown foreign nationals made expenditures during the election in violation of American election law.
Warner, the top Democrat on the Senate Intelligence Committee, said the suspected Russian placement of ads may have gone far beyond what Facebook disclosed, and that Twitter Inc and other tech companies should also examine the issue.
His committee is among those investigating alleged meddling and whether members of Trump's campaign colluded with Moscow.
Television has been the backbone of political advertising for decades, and local U.S. broadcasters are required to disclose a wealth of details about the cost and schedules of commercials. The ads can be seen by anyone with a television provided they are aired in their markets.
Online advertising offered by Facebook and other platforms such as Twitter and Alphabet Inc's Google, though, often targets narrow, more carefully constructed audiences based on factors such as age, political preference or interests.
Facebook is especially valued by advertisers due to the level of data it collects on users, meaning ads can be targeted with high levels of detail.
Google, the search engine which also owns video-sharing website YouTube, said on Thursday it always monitors for abuse and that it had seen no evidence of an ad campaign like the one Facebook disclosed.
Warner said he was hopeful social media companies would not oppose legislation.
"Americans, when it comes to elections, ought to be able to know there is foreign-sponsored content coming into their electoral process," he said.
'TIP OF THE ICEBERG'
Facebook briefed U.S. lawmakers on Wednesday and turned over information about the ads to Robert Mueller, the special counsel leading his own investigation into alleged Russian interference, a source familiar with the matter told Reuters.
That information included copies of the ads and data about the buyers, the source said.
Twitter is expected to brief congressional investigators soon on whether Russia used its advertising platform during the election, Warner told reporters. Twitter declined to comment.
Facebook Chief Executive Mark Zuckerberg said after the election last November that accusations his platform was manipulated to spread "fake news" to hurt Clinton's candidacy was "a pretty crazy idea."
Since then, Facebook and other tech firms have attempted to do more to address deliberately false stories or manipulation of traffic by bots.
"In the case of Facebook, they denied that they were being used in any way. They didn't do anything," Warner said at the security conference. "But by the time of the French elections, Facebook was working with the French" and shut down 50,000 accounts, he added.
Facebook's briefing to lawmakers on Wednesday "was the tip of the iceberg," Warner said. (Reporting by Dustin Volz and Jonathan Landay additional reporting by David Ingram in San Francisco; editing by Jonathan Oatis)