An explosion damaged a bridge that is one of the few links between Crimea and Ukraine’s mainland, Russia-backed officials said on Thursday, as the early stages of a Ukrainian counteroffensive unfold.

Russian forces in Crimea—which Russia annexed in 2014—use the Chongar bridge to reach southern Ukraine’s Kherson province. Russia and Ukraine control different sides of the province, which is a focus of fighting and attacks as Ukrainian troops try to reclaim Russian-occupied territory. Ukrainian authorities have said that for the country’s nearly 16-month war to end, Moscow must withdraw its forces from Crimea as well as from areas they seized during last year’s full-scale invasion.