A secretive breaking news feature being tested on a small number of Twitter users broke cover this week, as the service began issuing alerts through Twitter's smartphone app in the wake of the Berlin Christmas market attack.

Hours later, the breaking news service was used again to inform Twitter users that the Electoral College had voted to confirm Donald Trump as the next president of the United States.

In both cases the news was broken by Twitter using a notification in the same style as those used by BBC News, Sky News, Reuters and more. Tapping the notification and opening the Twitter app took users to 'Moments' relevant to each event. Moments are collections of tweets from reputable sources and eyewitnesses, gathered together to tell the bare bones of a news report.

Each Moment begins with a headline, followed by a couple of sentences to explain the story, then tweets, mostly with embedded images and video, of what has happened. In the case of the Berlin attack, where a truck was driven through a crowded Christmas market, killing at least 12 people and injuring dozens more, the Moment said: "Police confirm 12 deaths and at least 48 injuries after a truck plowed through a Christmas market. A suspect is in custody and the passenger of the vehicle died at the scene."

Most tweets used in Moments like this come from news agencies and journalists with verified Twitter accounts, or from eyewitnesses whose tweets have been retweeted by journalists and news publications.

These breaking news notifications join an alerts system already used by Twitter to inform users of extreme weather events nearby and local emergencies. But these have come through third parties, rather than be issued by Twitter itself. News alerts will force Twitter to exercise some editorial judgement regarding how and when the alerts are used, and exactly what the notification says.

Facebook also activated its Safety Check feature for users in Berlin to let their friends and family know they are safe. The feature is regularly activated during natural disasters and major incidents like terrorist attacks.

Twitter has confirmed the alerts are part of an ongoing trial which has been taking place among a small group of users for months, but knowledge of the feature only became widespread in the wake of 19 December and the day's two major news stories.

The alerts are targeted at different groups of users based on what Twitter perceives their interests to be, according to Mashable, which has spoken to a Twitter representative about the feature. IBTimes UK has also asked Twitter for comment and will update this story when we get a reply.

There is no set rule on how the alerts work, such as when they are sent out and to which users, although Twitter says past alerts have focused on Olympic results, the US election night, and softer news stories involving television programmes.