KHERSON, Ukraine >> Authorities rushed to rescue hundreds of people stranded on rooftops and supply drinking water to areas flooded by a collapsed dam in southern Ukraine on Wednesday, in a growing humanitarian and ecological disaster along a river that forms part of the front line in the 15-month war.

The collapse of the Kakhovka hydroelectric dam and emptying of its reservoir on the Dnieper River added to the misery the region has suffered for more than a year from artillery and missile attacks.

With humanitarian and ecological disasters still unfolding, it’s already clear that tens of thousands of people have been deprived of drinking water and many are homeless.

Some residents of Russia-occupied areas hit by high water complained that help was slow in arriving, with some stranded on roofs and streets passable only by boat in scenes more like natural disasters than wars. Others refused to leave.

The first report of casualties from the disaster emerged, with a mayor reporting three dead. At least 4,000 people have been evacuated from both the Russian and Ukrainian-controlled sides of the river, officials said, with the true scale of the disaster yet to emerge in an affected area that was home to more than 60,000 people. Russia-appointed authorities in the occupied parts of the Kherson region reported 15,000 flooded homes.

The dam and reservoir, essential for fresh water and irrigation for southern Ukraine, lies in the Kherson region that Moscow illegally annexed in September and has occupied for the past year.

The reservoir is also critical for water supplies to the Crimean Peninsula, which Russia illegally annexed in 2014.

Ukraine holds the Dnieper’s western bank.