In a battle against fake news, Facebook has started to roll out a new tool that fact-checks the stories a user shares.According to a report in Telegraph on Monday, some users in the US saw a pop-up window appearing when they shared a story on their profile, reading "Disputed by multiple, independent fact-checkers."Facebook has been severely criticised for promoting fake news and spreading misinformation. It was also accused of helping US President Donald Trump win the elections by allowing misinformation and fake stories on its platform."Before you share this content, you might want to know that the fact-checking sites, Snopes.com and Associated Press disputed its accuracy," the report noted.However, the tool is not available to all Facebook users yet. The pop-window appeared on a story about "Irish slave trade" which the Facebook fact-checking team believes is a conspiracy theory.To curb the fake news, Facebook has rolled out several tools to flag false stories. In December last year, social media giant announced to flag stories with the aim to provide more context to help people perceive the news in a proper way."We'll use the reports from our community, along with other signals, to send stories to these organisations," Facebook was quoted as saying.