States of emergency have been declared in parts of Bosnia and Serbia after the heaviest rain and worst floods since records began 120 years ago.
Overflowing rivers have burst into towns and villages, cutting off whole communities, while landslides have buried houses.
At least three people have drowned.
Army helicopters have been sent to evacuate thousands of stranded residents but reports say bad weather is hampering the rescue efforts.
Serbia's Prime Minister Aleksandar Vucic said it was the "greatest flooding disaster ever".
He said his government had appealed for help from the European Union, Russia and neighbouring countries.
Rescuers in Serbia say they have so far managed to to evacuate around 4,000 people.
In Bosnia, army helicopters have been trying to rescue residents, some of whom have been sitting on their roofs waiting for help.
But reports say strong wind and rain may be hampering the efforts.
"We had to abort the mission because of the rainfall and winds. The weather is horrible and the helicopter has its limits," said rescue pilot Fahrudin Memic.
"This is the most difficult mission I have ever participated in."
Thousands of homes are without electricity.
Are you in Bosnia or Serbia? Have you been affected by the floods? If you would you like to share your experience with the BBC you can email email@example.com, using the subject line 'Bosnia and Serbia floods'.