Borrowed from German Karst. The German term and the Slovene placename Kras (the Karst Plateau) are from Proto-Slavic *korsъ, from Italo-Dalmatian carsus (cf. Italian carso), possibly from Proto-Indo-European *ker- (“hard; rock”). More at Karst.
The metathesis in the Slovene term precludes German borrowing from Slovene.