Giant chunks of melting ice broke free on the Danube River on Monday, crashing hundreds of boats into each other, sweeping away several barges and sinking one of Belgrade's trademark floating nightclubs.

The thick ice had closed large parts of Europe's busy 2,860-kilometre-long waterway as the region faced a record-breaking cold snap, but it started shifting Sunday afternoon because of rising temperatures. There were no reports of injuries.

Officials around the region were trying to determine whether melting snow and ice would cause heavy flooding, adding more misery to an area that is dealing with a death toll that has risen over 600 and trapped thousands in remote, mountain villages behind massive snow banks.

Ice floes up to half-a-metre thick began to break up in the Belgrade area of the Danube, damaging boats and crashing pontoon bridges. A barge restaurant, called Alexander, sank completely, officials said.

The flashy floating restaurants and nightclubs lining the Danube are one of the main tourists attractions in the Serbian capital, by night blasting loud turbo folk music, a mixture of traditional Balkan folk and contemporary electronic beats.

Man killed by falling icicle in Romania

Officials were hoping that lowered water levels in the Danube from a drought last year would help Serbia avoid flooding like that that has hit Bulgaria and Greece.

Nikola Marjanovic, a Serbian water supply system official, said there is no need for panic but the situation must be carefully monitored.

"At this moment we don't know what will happen," he said.

Some 3,300 people remain stranded by deep snow that hit remote areas of southern Serbia and they can only be reached by helicopters, said Serbian emergency official Predrag Maric.

In Romania, melting snow and ice claimed the life of a 42-year-old man who was killed after an icicle fell on his head in front of the shop where he worked in the eastern city of Barlad.

In Montenegro, the U.S. military responded to a request from the government by dispatching two Black Hawk helicopters to evacuate people from snowbound regions of the small Balkan country, the U.S. Embassy said.