The worst floods to hit the Balkans in more than a century have killed dozens, and there are fears that number could rise as a major river is set to be hit by a new flood wave this evening.
Tens of thousands have fled their homes in Bosnia and Serbia after three months of rain fell on the region in just three days.
Thousands have also been evacuated in Croatia, where one person has died and two remain missing.
There are conflicting reports on the death toll. The AFP news agency is reporting 44 people have been killed, while Reuters puts the figure at 37.
People receive food in the flooded town of Obrenovac
Boats and helicopters have been used to evacuate those affected by floodwaters, which reached the second floor of homes in some parts of Bosnia.
Security Ministry spokesman Admir Malagic said approximately a million people, more than a quarter of the population, live in the affected area.
Bakir Izetbegovic, the chairman of Bosnia's three-man presidency, said the country was facing a "horrible catastrophe".
In the eastern town of Bijeljina, around 100,000 people were evacuated on Saturday after flood defences were unable to hold back the rain-swollen Sava River.
Boats and helicopters are being used to rescue people
The rain has also caused almost 300 landslides, burying dozens of houses and cars.
Many have lost homes they have only just finished rebuilding after the 1992-95 war which killed 100,000 people and devastated the country.
Mines from that conflict litter Bosnia's mountains, and many warning signs have been swept away, increasing the risk of deadly accidents.
Zeljka Cvijanovic, Prime Minister of Bosnia's autonomous Serb Republic, compared the devastation to the war, telling reporters: "The damage is such that we cannot recall even after the war."
Many people have lost homes rebuilt after the 1992-95 war
More than 15,000 people have been evacuated in Serbia.
Most of those who have fled their homes have found shelter in schools and sports halls.
Soldiers and emergency crews are using boats and helicopters to rescue thousands of people trapped in the town of Obrenovac, near the capital, Belgrade.
The flooding there is threatening the country's biggest power plant, Nikola Tesla. Sandbag defences are holding at the moment but another flood wave threatens the plant.
A man peers over at the high level of the Sava river in Sremska Mitrovica
Capacity at the plant has already been cut after a coal mine nearby was submerged.
Residents of the nearby town of Baric have also been ordered to leave immediately, with many leaving on buses and military trucks.
Hundreds of residents and soldiers have been hurriedly raising sandbag barriers around the Kostolac power plant to the east of Belgrade in a bid to divert waters from the River Mlava.
There was a slight respite for some parts of the country on Saturday when the rain eased, but Prime Minister Aleksandar Vucic warned a new flood wave on the Sava will hit this evening.
"The situation is catastrophic," he said.
People build a dam of sandbags by the Sava river
Thousands of volunteers have been bused in from all over the country after responding to a government appeal to help build flood defences along the river.
Residents have used social media to help collect food, blankets and clothes for crisis-hit areas.
Both countries have appealed for international help and many European Union countries have sent equipment and emergency crews.
A Russian team has joined the rescue effort and a team from the UK was expected to arrive by the end of the day.