Former president Barack Obama warned Facebook Inc. (FB) founder and CEO Mark Zuckerberg to take the threat of fake news more seriously, just over one week after the spread of dubious political advertising across social media helped Donald Trump to win the U.S. presidential election.

According to the Washington Post, Obama and Zuckerberg’s conversation took place at a gathering of world leaders in Lima, Peru, 11 days after Trump won the presidency. During the event, Obama pleaded with Facebook’s founder to make greater efforts to police fake news, warning that a failure to do so could impact presidential races for years to come. (See also: Facebook Shareholders Share Ire Over Fake News.)

Obama was reportedly disappointed with the nature in which Russia’s use of social media benefited Trump’s campaign and believed that Facebook should have done more to prevent the spread of political disinformation orchestrated by some of the country’s operatives. According to people briefed on the exchange, Zuckerberg acknowledged that fake news was a problem, but also told Obama that the messages were difficult to police and not widespread on Facebook's website.

It has since emerged that Facebook contacted the FBI in June 2016 after discovering elements of the Russian information operation on its website. The Washington Post reported that the company and government spent months trying to fix the issue, but were unable to find a suitable solution.

Over one year later, Facebook publicly admitted that it was manipulated in the lead up the presidential election. In an announcement, published via a Facebook Live video, the social media giant introduced new measures to better police political ads in the future after confirming that it had turned over 3,000 Russia-linked ads to congressional investigators.

Under the new rules, Facebook advertisers will be forced to disclose their identity when buying political ads — currently, advertisers are required to make disclosures for television, but not online. The company hopes these actions will bring greater transparency to its 2 billion-strong user base.

“I don’t want anyone to use our tools to undermine democracy,” Zuckerberg said in the video. “That’s not what we stand for. The integrity of our elections is fundamental to democracy around the world.” (See also: Peter Thiel Could Spell Trouble for Google, Amazon: Report.)

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