Facebook says it's discovered that an operation likely based in Russia spent $US100,000 ($A124,980) on thousands of US 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 polarising views on topics including immigration, race and gay rights.
Another $US50,000 was spent on 2,200 "potentially politically related" ads, likely by Russians, Facebook said.
US election law bars foreign nationals and foreign entities from spending money to expressly advocate the election or defeat of a candidate. Non-US 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 grey 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 US 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 US election, including potential collusion between the campaign of President Donald Trump and Moscow.
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 $US1 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 US intelligence agency conclusions that Russia was actively involved in shaping the election.
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."
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.
