The BBC's Guy Delauney says forecasters are warning that the River Sava is set to peak again in Serbia on Sunday evening
Emergency workers in Serbia are preparing for a "flood wave" on one of the country's main rivers.
Water levels on the River Sava are expected to peak later, threatening the country's biggest power plant.
It comes after the worst floods in the Balkans for decades left at least 20 dead and forced tens of thousands of people to flee their homes.
In Bosnia-Hercegovina, landslides have buried houses and disturbed landmines laid during the war in the 1990s.
The floods are also affecting Croatia.
Aerial footage showed flooding near Serbia Tesla power plant
Three months' worth of rain fell on the region in just a few days, causing rivers to burst their banks and sweep into people's homes. The death toll is expected to rise.
The rain has finally stopped in Belgrade, reports the BBC's Guy De Launey in the Serbian capital.
But officials fear water flowing from neighbouring Croatia and Bosnia could cause a "flood wave" on the River Sava on Sunday evening.
Emergency workers and volunteers have stacked sandbags along the river in Belgrade and other towns such as Sabac and Sremska Mitrovica.
Analysis: Guy De Launey, BBC News, Belgrade
The extreme weather in Bosnia is heaping danger upon misery. The floods are not just the worst in living memory - they are bringing a deadly reminder of the conflict of the 1990s back to the surface: Landmines.
There are still more than 100,000 landmines littering the Bosnian countryside, according to the country's Mine Action Centre. Almost two decades on from the end of the conflict, people are still killed and injured every year.
Casualty numbers have reduced over the years, thanks to demining efforts and the mapping of minefields. Local people know the danger areas - and the safe paths.
But the torrential rain, floods and landslides are likely to cause the mines to move. Warning signs have already been washed away in a number of places. When the floods finally recede, there will be an urgent need for the deminers to move in.
"We are doing all we can," said Serbian Prime Minister Aleksandar Vucic.
The main concern is whether flood waters could knock out the Nikola Tesla power plant, which supplies much of the country, our correspondent says.
It is in Obrenovac, south-west of Belgrade, where most of the population have already been moved to safety.
'Three metres high'
Serbian tennis player Novak Djokovic has joined calls for help.
Officials have refused to give a death toll for Obrenovac until the floodwaters recede.
Almost one-third of Bosnia has been affected. The north-east is worst-hit, with houses, roads and railway lines submerged.
Bosnian Serb police chief Gojko Vasic said the situation had been particularly difficult in Doboj "because the flood waters acted as a tsunami, three to four metres high", the Reuters news agency reported.
"No-one could have resisted," he said.
Authorities have warned that the number of fatalities across the Balkans could rise as thousands of homes remain without electricity or drinking water and scores - like this man - are evacuated
Almost one-third of Bosnia is under water - from the air the north-eastern corner of the country resembles a huge muddy lake
In Serbia the floodwaters have been described as similar to a tsunami, three to four metres high in places
The flooding has also caused considerable infrastructure damage throughout the Balkans
'Horrible catastrophe'
Bosnian Security Ministry spokesman Admir Malagic said that about one million people - more than a quarter of the country's population - live in the affected area.
Chairman of the Bosnian three-man presidency Bakir Izetbegovic said that his country is facing a "horrible catastrophe", the Associated Press reported.
"We are still not fully aware of actual dimensions of the catastrophe," he said.
