Russian troops defending a pocket of territory wedged between a river and the border with Ukraine were at risk of becoming encircled, military analysts said Monday, after Ukraine bombed bridges that are the only routes for resupply or retreat.

In their counterattack into Russia, which has been underway now for nearly two weeks, Ukrainian troops quickly broke through thinly manned border defenses, fanned out on highways and captured towns and villages, initially pushing deeper into Russian territory.

The bombing of bridges, in contrast, takes aim at land between the Seym River, the border and an area inside Russia already controlled by Ukraine, with the potential to entrap the Russian forces positioned there. Three bridges span this stretch of river, all now destroyed or damaged, according to statements released by the Ukrainian Air Force and to social media posts by Russian officials and military commentators.

“Minus one more bridge!” the Ukrainian Air Force commander, Lt. Gen. Mykola Oleshchuk, wrote in a post on Telegram on Sunday.

The potential encirclement of its forces in the area adds another challenge to a Russian Army caught off guard by Ukraine’s startling incursion over the border on Aug. 6. The operation has injected a new sense of optimism to Ukrainian forces that had been backpedaling for months elsewhere along the front line.