Croatian emergency crews and soldiers are struggling to contain the swollen Sava River as it reached record levels south-east of the capital, Zagreb.
Authorities said water levels in the town of Jasenovac have exceeded the highest-ever recorded by some 4in (10cm). About a dozen houses in a nearby village are cut off.
The Croatian state TV channel HRT said residents have refused to evacuate, so emergency crews are delivering food and water by boat.
Experts said the Sava is expected to rise further in the coming days due to fresh snow.
To the east, snow and freezing rain have delayed dozens of flights and some trains in Romania amid an unseasonal cold snap.
Valentin Iordache, the spokesman for Bucharest airports, reported 30 flight delays on Monday morning due to the wintry weather.
Temperatures were around minus 5C (23 Fahrenheit). Trains running from Bucharest to the Black Sea port of Constanta and the southern city of Craiova were also delayed.
Weather forecasters issued an amber warning on Monday for Bucharest and seven counties in southern and eastern Romania for sleet, snow and black ice.
Winter snow has also hit Germany, Hungary and the UK, among other European nations.