Facebook is rolling out its new tool against fake news.。

SEE ALSO:Facebook is cracking down on hoaxes in your News Feed。

The feature, announced in December is the result of collaboration with "third-party fact-checking organisations." 。

Designed to combat to the spread of misinformation on the platform, it warns users when they try to post fake stories. 。

The fact-checker is working for some U.S.-based users who tried to share a story that falsely asserted thousands of Irish people were brought to the United States as slaves.
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The story entitled "The Irish slave trade -- the slaves that time forgot" was published by the Rhode Island blog Newport Buzz and widely shared ahead of St Patrick's Day on 17 March.
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Attempts to share the story triggered a red alert in the pop up window stating that the article has been disputed by Snopes.com and。 the Associated Press
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Clicking on it would open a second pop-up with more information:。

"Sometimes people share fake news without knowing it. When independent fact-checkers dispute this content, you may be able to visit their websites to find out why,” it reads. “Only fact-checkers signed up to Poynter’s non-partisan code of principles are shown.”Mashable Light SpeedWant more out-of-this world tech, space and science stories?Sign up for Mashable's weekly Light Speed newsletter. 。Sign up for Mashable's weekly Light Speed newsletter. 。By signing up you agree to our Terms of Use and Privacy Policy.。

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It also links to Facebook's official help page and to the debunking articles from AP and Snopes.com.。

If you ignore the warning, another pop-up asserts that the article's accuracy is "disputed by multiple, independent fact-checkers".。

If you proceed to publish the article a red box still shows up in the timeline: 。

The feature was announced in December 2016 but this appears to be the first time people have noticed it on a story that went viral.。Mashable。

attempted to post the story using a device in London, but the red alert did not show up. 。

Facebook's help centre page entitled 'How is news marked as disputed on Facebook' confirms that the feature isn't available to everyone yet.。

Some Trump supporters have been critical of the tool, questioning its veracity.。

Paul Joseph Watson, editor-at-large of Infowars, criticised the inclusion of Snopes in Facebook's list of independent fact-checkers. 。

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Liam Hogan, a librarian and historian based in Ireland, said Trump supporters were "losing their minds" about the red alert:。

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Other Trump fans or right-wing bloggers took to Twitter to assert that the Irish slave trade was real and Facebook was committing "Pre-Thoughtcrime": 。

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