Germany's domestic intelligence agency has put a regional branch of the far-right Alternative for Germany (AfD) party under observation, the Interior Ministry said Monday.

The decision was made after long deliberation, the ministry said.

The Brandenburg branch of the AfD has provoked a rift within the party by voting to keep Andreas Kalbitz, a member who was expelled by the national party, as its regional chief.

The national party says it excluded Kalbitz based on his former membership of the now-banned right-wing extremist group, Heimattreue Deutsche Jugend (HDJ, or the German Youth Faithful to the Homeland.)

Kalbitz also led the Fluegel (Wing) group, a far-right nationalist faction within the AfD, alongside firebrand Bjoern Hoecke, which was officially dissolved earlier this year following a request by the AfD executive board.

Although the group's online presence no longer exists, the Office for the Protection of the Constitution – Germany's domestic intelligence agency – has no reliable evidence of an actual dissolution.

Monday's decision means that the Brandenburg branch of the AfD has been categorized as a "Verdachtsfall," German for "suspected case," giving the intelligence agency access to certain data and allowing it to deploy agents to monitor party events.

It could then choose to classify it as a case for full surveillance, in which case it could infiltrate the branch with confidential informants.

The AfD's surge in support in general elections in 2017 made it the country's third-strongest party and largest opposition force in the Bundestag, sending shockwaves through the political establishment.