Flooding concerns as melting ice in Danube causes chaos.
The thick ice had closed large parts of Europe's busy waterway as the region faced a record-breaking cold snap, but it started shifting on Sunday afternoon because of rising temperatures.
Officials were trying to determine whether melting snow and ice would cause heavy flooding, creating more misery in an area that is dealing with a death toll that has risen past 600 and trapped thousands in remote mountain villages behind massive snow banks.
The floating restaurants and nightclubs lining the Danube are one of the main tourist attractions in the Serbian capital. Officials were hoping that lowered water levels in the Danube from a drought last year would help Serbia avoid flooding like that which has hit Bulgaria and Greece.
Nikola Marjanovic, a Serbian water supply official, said there is no need for panic but the situation must be monitored.
"At this moment we don't know what will happen," he said.
Some 3,300 people remain stranded in remote areas of southern Serbia and can only be reached by helicopter.
* A man walks by damaged boats on a frozen part of the Danube river in Belgrade, Serbia