BERLIN (Reuters) – Germany’s domestic intelligence agency has put a regional branch of the Alternative for Germany (AfD) under surveillance, denting the nationalist party’s efforts to establish itself as a credible opposition force.

The move followed the AfD’s expulsion last month of Andreas Kalbitz, party chief in the eastern state of Brandenburg, over links with far-right groups. The AfD national executive committee’s decision is still disputed by rank-and-file members.

“The Brandenburg AfD has become more and more radical since its foundation and is now dominated by endeavours that are clearly directed against our free democratic fundamental order,” Brandenburg Interior Minister Michael Stuebgen said.

He said parts of the state branch were dominated by the ideas of a far-right group in the party known as ‘Der Fluegel’ (The Wing).

“In the Brandenburg AfD, the ‘wing’ has long been the whole bird,” Stuebgen told reporters in Potsdam.

The AfD condemned Monday’s decision.

“(It) is just as wrong as the previous classifications of the AfD by the Office for the Protection of the Constitution,” party Honorary Chairman Alexander Gauland said in a statement emailed to Reuters, using the official name of Germany’s BfV domestic intelligence agency.

Last year the BfV sought to classify the AfD as “a case to investigate” for possible unconstitutional activities.

But a court barred the agency from using the classification, saying it put the AfD at a disadvantage to other political parties. However, the court did not ask the agency to stop its surveillance of the party.

The AfD took second place last September in a regional election in Brandenburg, which surrounds the capital Berlin, building on its success in the 2017 national election when it came third to become the largest opposition party.

The AfD harnessed voter anger over the arrival of more than one million migrants in 2015 and the planned closure of coal mines in the formerly communist eastern states.

But it has seen popular support fall during the coronavirus pandemic, which centre-right Chancellor Angela Merkel is widely seen to have handled competently.