Meteorologists say the flooding is the worst since records began 120 years ago and is due to a three-month amount of rain that fell on the region in just three days. Goran Mihajlovic from Serbia's Weather Center told AP that such rainfall happens once in 100 years.
In the eastern Bosnian town of Bijeljina, some 10,000 people were being evacuated Saturday after the rain-swollen Sava River surged through flood defenses.
Officials in Bosnia say 12 people died and more bodies could emerge as the water recedes from dozens of cities flooded in the past three days. In some places, the water had reached the second floor of people's homes and they had to be rescued by helicopter from their roofs.
In Serbia, which saw eight deaths, emergency crews and soldiers were using boats and helicopters to rescue thousands trapped in the town of Obrenovac, near Belgrade. The overflowing waters there are now threatening Serbia's biggest power plant.
Prime Minister Aleksandar Vucic told a press conference that a new flood wave on the Sava River will hit Sunday evening.
Thousands of volunteers have responded to government's appeal to help build up flood defenses around the towns along the Sava.