The leadership of populist party Alternative for Germany (AfD) has issued an official warning to Andre Poggenburg on Friday after comments found in an internal WhatsApp chat leaked last week.

Party leaders Alice Weidel and Alexander Gauland issued the official warning, telling the Deutsche Presse Agentur his comments "massively damaged" the party's image and "pushed the party closer to right extremism."

AfD leader Alice Weidel says blunt slogans do not help the party

The left-wing news site "linksunten" published a WhatsApp group chat last Monday containing AfD members, called "AfD Info LSA." Federal police and domestic intelligence is combing through the chat dated from February to May of this year.

Read More: Ten things you need to know about the AfD

Poggenburg was found using the slogan "Deutschland den Deutschen" (roughly: "Germany for the Germans") - a phrase commonly used by neo-Nazi groups in Germany and often used by the far-right NPD party. The AfD warned its deputy chairman in Mecklenburg Western-Pomerania, Ralph Weber, in April for using the same slogan on Facebook.

AfD leaders have sought to distance themselves from the NPD.

Facing initial criticism last week after the leak, Poggenburg at first defended his use of the slogan in a tweet: "Why should this put pressure on me? Of course 'Germany belongs to the Germans,' and that's how it should stay!"

He also suggested AfD members receive a seminar on "Erweiterung der Aussengrenzen" ("expanding external borders"), a suggestion that another unnamed WhatsApp group member responded with thumbs up signs and flexed biceps emojis. Leadership has condemned this exchange as well, saying it was inconsistent with the AfD's platform.

He has said he was referring to securing Europe's borders.

Weidel, AfD's candidate for chancellor along with Gauland in Germany's election later this year, said the AfD sees itself as a "party for political realism" and "blunt sayings do not help [this image] and hurt the party. Whoever doesn't understand that has no place at the AfD."

dv/rc (AP, dpa)