By Joseph Menn and David Ingram

SAN FRANCISCO (Reuters) - Facebook Inc said on Wednesday it had found that an operation likely based in Russia spent $100,000 on thousands of U.S. ads promoting divisive social and political messages in a two-year-period through May.

Facebook, the dominant social media network, said 3,000 ads and 470 "inauthentic" accounts and pages spread polarizing views on topics including immigration, race and gay rights.

Another $50,000 was spent on 2,200 "potentially politically related" ads, likely by Russians, Facebook said.

U.S. election law bars foreign nationals and foreign entities from spending money to expressly advocate the election or defeat of a candidate. Non-U.S. citizens may generally advertise on issues. Other ads, such as those that mention a candidate but do not call for the candidate's election or defeat, fall into what lawyers have called a legal gray area.

Facebook announced the findings in a blog post by its chief security officer, Alex Stamos, and said that it was cooperating with federal inquiries into influence operations during the 2016 U.S. presidential election.

Facebook briefed members of both the Senate and House of Representatives intelligence committees on Wednesday about the suspected Russia advertising, according to a congressional source familiar with the matter. Both committees are conducting probes into alleged Russian interference in the 2016 U.S. election, including potential collusion between the campaign of President Donald Trump and Moscow.

Facebook also gave its findings to Robert Mueller, the special counsel in charge of investigating alleged Russian interference in last year's presidential election, a source familiar with the matter said. The company produced copies of advertisements as well as data about the buyers, the source said.

Mueller's office declined to comment.

Facebook said it found no link between the Russian-purchased advertising and any specific presidential campaign. The ads were mostly national in their focus and did not appear to reflect targeting of political swing-states, the company said.

Even if no laws were violated, Facebook said the 470 accounts and pages associated with the ads ran afoul of the social network's requirements for authenticity and have since been suspended.

Facebook did not print the names of any of the suspended pages, but some of them included such words as "refugee" and "patriot."

More than $1 billion was spent on political ads during the 2016 presidential campaign, thousands of times more than the presumed Russian spending identified by Facebook's security team.

But the findings buttress U.S. intelligence agency conclusions that Russia was actively involved in shaping the election.

Facebook previously published a white paper on influence operations, including what it said were fake "amplifier" accounts for propaganda, and said it was cracking down.

As recently as June, Facebook told journalists that it had not found any evidence of Russian operatives buying election-related ads on its platform.

"TROLL FACTORY" CONNECTION

Representative Adam Schiff, the top Democrat on the House Intelligence Committee, called the Facebook report "deeply disturbing and yet fully consistent with the unclassified assessment of the intelligence community."

"We are keenly interested in Russia's use of social media platforms, both the use of bots and trolls to spread disinformation and propaganda, including through the use of paid online advertising," he said in a statement.

A Facebook employee said Wednesday that there were unspecified connections between the divisive issue ads and a well-known Russian "troll factory" in St. Petersburg that publishes comments on social media.

Ellen Weintraub, a member of the U.S. Federal Election Commission, said U.S. voters deserve to know where the ads are coming from and that the money behind them is legal.

"It is unlawful for foreign nationals to be spending money in connection with any federal, state or local election, directly or indirectly," Weintraub said in a phone interview.

She declined to comment on the Facebook ads, saying she could not discuss subjects that could come before the agency.

Facebook declined to release the ads themselves, prompting a sharp rebuke on Twitter from Pierre Omidyar, the billionaire founder of First Look Media, a producer of feature and documentary films, television and podcasts.

"Facebook keeps the targeted political ads it publishes secret, emboldening criminals," wrote Omidyar, the eBay founder who also provided funding to launch media organization The Intercept. "I don't see how that can possibly be legal."

Facebook's disclosure may be the first time a private entity has pointed to receiving Russian money related to U.S. elections, said Brendan Fischer, a program director at the Campaign Legal Center, a Washington nonprofit that advocates for more transparency.

"Whoever may have provided assistance to Russia in buying these Facebook ads is very likely in violation of the law," he said, adding that Facebook has a legal duty to act if it is aware of similar activity in the future.

(Reporting by Joseph Menn and David Ingram; Additional reporting by Dustin Volz in Washington; Editing by Jonathan Weber and Leslie Adler)