By 9 September, the counteroffensive in Kharkiv Oblast led to a breakthrough.[1] By 10 September, the Institute for the Study of War said that Ukrainian forces had captured approximately 2,500 square kilometres (970 sq mi) in the Kharkiv region by effectively exploiting the breakthrough,[2] and Reuters reported that Russian forces had been forced to withdraw from their base at Izium after being cut off by the capture of the key railway hub Kupiansk.[21] On 13 September, Ukrainian forces established a bridgehead over the Oskil River near Borova.[22] By 17 September, it was confirmed that Ukrainian forces had breached the Russian front line along the Oskil River.[23] On 17 September, Ukrainian forces claimed that they had advanced to the left bank of the Oskil River.[24]