Germany's domestic intelligence agency, the BfV, is to start monitoring the far-right AfD party in a move endorsed by the government, but decried as a political witch-hunt by the party's leaders.

"The BfV has initial indications that the AfD's policies are against the democratic constitutional order," Thomas Haldenwang, the BfV head, told press in Berlin on Tuesday (15 January).

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"We will take legal action", the AfD's Alexander Gauland said (Photo: Reuters)

"But those indications are not sufficiently concentrated to start monitoring the party using espionage methods," he added.

He classified the AfD as a "case to investigate" and said monitoring would be based on open sources.

But the project could lead to full surveillance if the AfD was later deemed "suspicious" and "a threat to the liberal democratic principles of Germany's constitution".

Full surveillance would already be deployed against two of the AfD's offshoots - The Wing and Young Alternative - the BfV added, however.

The Wing is an extreme-right group led by the AfD's regional boss in Thuringia, Bjoern Hoecke.

Its aim was to "marginalise, disparage, and disenfranchise foreigners, migrants, especially Muslims" and to play down Nazi crimes, the BfV said in a statement.

The Young Alternative is the AfD's youth branch and is suspected of having ties with the Identitarian Movement - a white-supremacist group, which is already under full BfV surveillance.

The BfV spoke out on Tuesday after trawling through 182 speeches by 50 AfD members, including 80 hours of video footage and 1,069 pages of text, as well as Facebook material, over the past four months.

Regional security services in Bremen, Lower Saxony, and Thuringia had also investigated the AfD in recent months.

The BfV move was endorsed by German foreign minister Heiko Maas and interior minister Horst Seehofer.

"Whoever discriminates against people on the basis of their origins is being racist and nationalist. Parts of the AfD are a case for the domestic intelligence office," Maas said.

"We also assessed this extensive study ourselves. We think it's plausible and that is why I support these decisions," Seehofer said.

Seehofer conceded that the BfV step had "political importance", in terms of stigmatising the AfD in the eyes of mainstream conservative voters.

"But I attach great importance to the fact that this is not a decision made by politicians, but made by the constitutional protection authorities," he added.

The AfD said it would challenge the BfV in court.

"We will take legal action against this decision," AfD co-leader Alexander Gauland said, blaming the move on "political pressure".

"With Mr Maassen this decision would not have been possible at all, so he had to go, so they created these hunts, and now we have this decision," Alice Weidel, another AfD co-leader said.

Hans-Georg Maassen was Thomas Haldenwang's predecessor as BfV head.

But he was forced out of his post last year after denying reports of racist riots in the German city of Chemnitz and meeting with AfD leaders in what he called his personal, or "ex-officio", capacity.

The AfD's Hoecke, who previously called Germany's Holocaust memorial a "monument of shame" and who said it should make a "180-degree turnaround in [its] politics of [WW2] remembrance", also attacked the investigation.

"I'm already sorry for the officials who have to kill their time looking for things that do not exist," he said.

The AfD started out in 2013 in opposition to Germany's bailout of Greece.

It entered the German parliament in 2017, winning 94 out of 709 seats to become the country's third largest party.

It is also represented in all 16 of Germany's regional parliaments.

It has just one MEP, but is polling to win up to 13 out of Germany's 96 European Parliament (EP) seats in the European elections in May, with a manifesto to quit the euro and to dissolve the EP, among other ideas.

"Observing them won't solve the problems. Above all, we need to deal with the AfD objectively and politically," Maas, the German foreign minister, added on Tuesday.

The BfV move came after an AfD politician, Frank Magnitz, was beaten up last week in the city of Bremen in what police called a politically-motivated attack.

German intelligence has in the past also monitored Die Linke, a German far-left party.