As promised, Facebook has introduced a 'red-flag' third-party, fact-checking tool to counter the spread of fake news, *The Guardian* reported on Wednesday.

However, the feature has reportedly been rolled out on a trial basis and may not be immediately available to most users.

The company had said in January that it will start working with independent fact-checkers to identify fake news and tag such stories with a warning.

The tool was first seen by users who tried to share a fake story about thousands of Irish people being brought to the US as slaves, published by the Rhode Island entertainment blog Newport Buzz, The Guardian reported.

According to the publication, a warning popped up while some users tried to share the story, informing them that the facts inside it were disputed by Snopes.com and the Associated Press.

Upon clicking the warning, the post expanded and read: “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.”

“Only fact-checkers signed up to Poynter’s non-partisan code of principles are shown,” the warning further said.

However, the warning sign did not appear all over the world and while attempts made by The Guardian in San Francisco to publish the Newport Buzz story triggered the tool, the warning did not appear in Sydney or London, the report said.

The article further said that it was also possible to flag it to Facebook as a 'fake news story' through the usual reporting process.

On 16 March, the Associated Press had published a 'Fact Check' on the so-called “Irish slave trade” that rejected “the false articles, trending on social media”, which was termed to be 'a part of an ongoing Associated Press effort to fact-check claims in suspected false news stories', The Guardian said.

It is not unusual for Facebook to trial new features on a small number of users before applying them across the board.

Facebook declined to comment on the roll-out of the tool, but its help centre page on how news is “marked as disputed on Facebook” confirmed “this feature isn’t available to everyone yet”, The Guardian said.