Users will also be able to report potentially false stories to Facebook, or send messages to the person posting a questionable article directly.
The company is focusing, for now, on what Mosseri called the "bottom of the barrel" websites which are purposefully set up to deceive and spread fake news, as well as those that are impersonating other news organisations. "We are not looking to flag legitimate organisations," Mosseri said. "We're looking for pages posing as legitimate organisations."
Articles from legitimate sites that are controversial or even wrong shouldn't get flagged, he said.
There is no blacklist of sites that will automatically have their stories sent on, Mosseri said.
But Facebook has built a sort of data profile of characteristics fake news articles share - such as low share numbers after the headline is clicked - which it will use to decide when to have something fact-checked.
The company will also prioritise checking stories that are getting lots of flags from users and are being shared widely, to go after the biggest targets possible.
If someone wants to appeal a label, they can direct that complaint to the fact-checking organisation that made the call on whether an article was true or not.
Facebook is also trying to crack down on people who've made a business going into fake news by tweaking its advertising practices. Any article that's been disputed, for example, cannot be used in an ad.
Facebook's also playing around with ways to limit links from publishers with landing pages that are mostly ads - a common tactic for fake news websites.
With those measures in place, "we're hoping financially motivated spammers might move away from fake news," Mosseri said.
All of these efforts, Mosseri said, are works in progress. Users will start seeing them soon, but Facebook is testing out different options to see what works best. This round of efforts is definitely the first of many that Facebook will have to try to stay ahead of fake news sites.
"We don't think it will get us all the way there," he said. "I expect it to be something we need to invest in on an ongoing basis."