Water levels on the River Sava are expected to peak in the next few hours, prompting emergency workers in Serbia to prepare for a “ flood wave”.
Efforts have been made to protect the country’s biggest power plant from the surge.
The flooding is affecting countries right across the Balkan region, with three months worth of rain having fallen in just a few days.
The death toll is said to risen to at least 44 with fears that it may well go higher as the waters recede and more bodies are discovered.
On the Bosnia -Croatia border, stranded victims who are prepared to leave their homes are being picked up. Although the rain eased on Sunday there, the lack of power and drinking water is making life impossible.
Almost one third of Bosnia has been hit by the floods with tales of a tsunami of water up to three metres deep sweeping into some villages.
A massive relief programme is underway with donations of clothes and food pouring into collection centres in most major towns.
Evacuees – many shocked by having lost everything – are being housed in schools and community halls and are being cared for by thousands of volunteers from across Europe.