Germany's domestic spy agency will investigate Alternative for Germany (AfD) to see whether the anti-immigrant party's policies breach constitutional safeguards against extremism, its chief said on Tuesday.

The BfV has classified Germany's main opposition party as a "case to investigate," domestic intelligence chief Thomas Haldenwang said, but the probe would fall short of full-blown surveillance of the AfD.

Germany's constitution contains strict protections against extremism, allowing for the close monitoring and even the outlawing of far-left or far-right parties. The measure is rare, though the BfV has previously monitored other political parties.