Some 12,000 North Korean troops will be sent to Russia, a key United States ally said Thursday, vowing it “will not stand by and do nothing” in the face of this significant “provocation.”

Disguised as Buryats and Yakuts, ethnic minorities from Siberia who have made up a disproportionate share of Moscow’s forces, the North Korean troops are part of a 12,000-strong force sent to help Russia retake the Kursk region, partly held by Ukraine since August, according to video footage published by the South Korean intelligence service.