The breach of a huge dam on the front-line Dnipro River has muddied the picture for a much-awaited Ukrainian counteroffensive against Russian invaders and threatens an environmental disaster for civilians living in the war zone.

Kyiv and Moscow have blamed each other for the collapse of the Nova Kakhovka dam in southern Ukraine that sent floodwaters gushing toward surrounding towns and farmland and forced hundreds of civilians to flee on Tuesday.

With water levels still rising, officials and analysts have begun to count the human and environmental costs for one of the world's most fertile agricultural countries, saying settlements, thousands of people and some rare wildlife species are at risk.