Facebook Inc. has identified an ongoing effort to influence the U.S. midterm elections, using inauthentic accounts and pages on the social network.

“We’re still in the very early stages of our investigation and don’t have all the facts -- including who may be behind this,” Facebook said Tuesday in a blog post. “It’s clear that whoever set up these accounts went to much greater lengths to obscure their true identities than the Russian-based Internet Research Agency has in the past.”

The company has stepped up its work to ferret out fake accounts after a Russian-backed effort sowed discord among Americans during the 2016 presidential campaign. Facebook and other social-media networks have been criticized by lawmakers for failing to recognize and take steps to stop those efforts. The new disclosure underscores the increasing difficulty of distinguishing real political discourse from content meant specifically to create dissension among Americans, even as Facebook invests heavily in systems to spot and take down such posts.

Facebook said that starting last week, it identified eight pages and 17 profiles on its main social network, and seven accounts on Instagram, that violated its rules. It shared the findings with U.S. law enforcement, Congress, and other technology companies. The Menlo Park, California-based company said it was letting the public know ahead of a real protest the fake accounts had planned and coordinated in the nation’s capital for next week.

“At this point in our investigation, we do not have enough technical evidence to state definitively who is behind this,” Nathaniel Gleicher, Facebook’s head of cybersecurity policy, said on a conference call. “These accounts have engaged in some similar activity and in some cases have connected with known IRA accounts.” He said the full extent of this effort may not yet be known. "We’re following up on thousands of leads.”

On the call, Facebook executives said they are sharing the evidence they found with law enforcement, which can help make a determination about the groups’ motive and impact. The company said it only sees what happens on its platform, and is not able to judge what the campaign was aiming to achieve.

Democratic Senator Mark Warner, who serves as vice chairman of the Senate Intelligence Committee, said in a statement that he was "glad that Facebook is taking some steps to pinpoint and address this activity."

"I also expect Facebook, along with other platform companies, will continue to identify Russian troll activity and to work with Congress on updating our laws to better protect our democracy in the future," said Warner, adding that he had received a briefing on the discoveries.

The U.S. intelligence community has occasionally suggested social media sites are better positioned to catch evidence of meddling on their platforms, while the companies have pushed back that it’s law enforcement that has the requisite expertise.

Asked about Facebook’s disclosures, Senator John Cornyn said he’s been briefed on efforts to impact U.S. politics via social media and noted the Intelligence Committee has a hearing Wednesday to hear from experts on the subject.

"It demonstrates again a huge vulnerability that we have where these people who claim to be somebody else can foment such conflict and dissension within our society," said Cornyn, the Senate’s No. 2 Republican. "It’s like pouring gasoline on a fire."

"Our adversaries are taking advantage of that," he said. "But it strikes me that it’s not just foreign states, it also can be political opponents."

The discovery of the election-interference campaign was earlier reported by the New York Times.