German Chancellor Angela Merkel on Thursday praised the "wonderful solidarity" of tens of thousands struggling against central Europe's worst river floods in over a decade and promised more help for the victims on her second visit in three days to disaster-hit areas.

Vast stretches along the Elbe river basin have been submerged in northeast Germany and upstream in the Czech Republic, with only trees and red-tiled roofs sticking out of the muddy water in many abandoned villages now accessible only by boat or helicopter.

The picture of devastation was similar along the mighty Danube, which has jumped its banks in Germany's southern Bavaria state and Austria and sparked large-scale disaster preparations in Hungary, where the water was expected to peak in coming days.

Merkel visited one of Germany's most flood-threatened cities, Bitterfeld in Saxony-Anhalt state, where two lakes, one higher than the other, loom dangerously close, threatening what locals called a "mini-tsunami" and forcing a mass evacuation.

"The people are scared and it's understandable," said Merkel, who was wearing khaki like the soldiers she shook hands with on the visit which comes four months ahead of elections.

"We can't undo the force of nature but I think people can count on everything humanly possible being done," she said, thanking young people who have organised aid efforts online as well as the 85,000 deployed firefighters, troops and aid workers.

"People here are facing a very difficult situation for the third time," Merkel said in Bitterfeld -- once a heavily polluted industrial centre in the former East Germany -- pointing to the communist state's 1990 collapse and the massive "floods of the century" in 2002.

Having promised 100 million euros ($130 million) in immediate aid on Tuesday, Merkel said more would be coming: "When the flood has retreated somewhat, we will have to work on an analysis of the total damage and then we will of course help ... we will do our part."

The day Merkel visited, an exhausted volunteer relief worker in his 60s collapsed and died near Magdeburg in the same eastern region -- a further casualty after the regional disaster claimed 12 victims, including eight in the Czech Republic.

Bitterfeld was just one of many riverside cities threatened by the swollen Elbe, which has risen from its usual two to above eight metres (six to above 26 feet) after days of torrential rains, putting heavy strain on saturated dykes.

Tens of thousands have been evacuated -- including 30,000 in the nearby city of Halle, which a day earlier reported the highest water level in 400 years on a local tributary.

Nonetheless, some residents remained defiant.

"I'm staying," said 77-year-old Bitterfeld lakeside resident Joachim Grollmitz. "I won't let them force me out. We still have electricity and water and can stay informed about the water level."

But he did admit to AFP to some concern about a possible breach between the two lakes, saying that "then a big wave would come and roll over us".

The mass mobilisation had its lighter moments.

In a scene reminiscent of the book and movie "The Life of Pi", a cheetah was taken to safety in a rubber dinghy from a nearby flooded zoo to a dry animal enclosure, now dubbed "Noah's Ark". The wild cat had been sedated.

Upstream in the Czech Republic -- where the floods have forced some 20,000 evacuations -- rescue workers in rubber dinghies were supplying isolated families who lack drinking water, power or gas.

In the industrial centre of Usti nad Labem, where 11,000 people were told to evacuate, looters targeted empty homes and businesses, and a waiter at a pub-restaurant told how he came face to face with three robbers at night.

"I entered the corridor and got a blow. They broke my nose, my side is sore and there's something wrong with my ribs," Ladislav Kratochvil told the DNES daily.

In Austria, where two people have died in the floods, the Danube town of Korneuburg just north of Vienna reported an all-time record river level of 8.06 metres.

Down the Danube in Hungary, preparations moved into high gear to prepare Budapest for the wall of water coming along one of Europe's longest waterways which empties into the Black Sea.

Prime Minister Viktor Orban has warned of "a real threat to human life" but has pledged that "with good cooperation, we can protect everyone".

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