Upstream states are afraid that the still-standing bridge pillars and huge blocks of concrete roadway lying just below the surface could trap ice floes and cause catastrophic flooding if they are not removed before winter sets in.
Zoltan Illes, chairman of the Hungarian parliament's environment committee, said: "Once the river is obstructed the ice can back up more than 100km and the water will overtop the banks in Hungary, as well as in Serbia and Croatia.
"We have only just realised the terrible danger. We thought when the war was over the first thing would be the clearing of the fallen bridges, but suddenly it has occurred to us that no one intends to do it before the winter, when the ice comes."
The emergency was highlighted at a floating conference on the Danube, focused on fostering cooperation to deal with the high level of pollution in the river, which rises in Germany and passes through 11 countries on its way to the Black Sea.
Mr Illes, a vice-president of the Hungarian government party Fidesz (Young Democratic party), said his colleague Kalman Katona, minister of transport and water management, was contacting the EU in Brussels for urgent help.
"We realise the EU cannot give money to Serbia, but our plan is to hire the only two big water cranes on the river, which are currently in Austria, and move the obstructions ourselves.
"We are asking the EU to lend us the money so we can do it. We can sort out who pays afterwards."
The problem was urgent, as winter would set in within the next six weeks, and icing could start in December, Mr Illes said.
Gyorgy Droppa, manager of the Danube Circle Environment Organisation, the largest green group in Hungary, said seven bridges had been bombed and an "ice cork" could form at any of them.
"Because the river drops only an inch or so for each mile it passes through Hungary and through to Belgrade, the ice backs up very far up river - a hundred kilometres if action is not taken quickly.
"The flow is very fast, especially in flood times, and disaster can come very quickly. In times past we have had to call in the army to blow up the ice if the breakers cannot cope."
Hungary has two specialist ice-breaking vessels which it uses on the river in winter to break up floes which lodge against obstructions in the river.
Mr Droppa is a member of the committee of European Green parties, which passed a resolution last week drawing attention to the danger of flooding.
"I was very surprised afterwards to be called by the American embassy, who asked me whether this was a serious problem," he said.
"I told them it was - certainly for us and for them in propaganda terms."
Mr Droppa, who runs a river engineering business on the Danube, said Hungary had built the ice-breakers in 1956 after terrible floods caused by ice corks had inundated thousands of homes. Hungary had an agreement with Serbia for them to operate well into Serbian territory to protect vulnerable cities.
"These include Bezdan in Croatia and Mohacs and Baja in Hungary, and as far upstream as Kalosca. It is impossible to say how many properties are at risk, but it is hundreds of thousands of people altogether."
Mr Droppa said one in three years was a bad ice year and one in 20 was very severe.
"Maybe twice in 10 years we have ice floes several centimetres thick and hundreds of metres long. Ice like that flattens islands in the river and slices through trees on the banks.
"If it hits an obstruction like a fallen bridge, it piles up as more and more ice is driven into it, rapidly forming a dam. The average flow of the Danube in Hungary is 2,000 cubic metres a second - that is a lot of water."
Miroslav Spasojevic, a Yugoslav environment ministry official, said it had recognised the problem.
"We have a temporary bridge in Novi Sad held up on barges for buses to cross, which we know would obstruct the ice too. We have decided to destroy this if the ice threatens, but we can do nothing about the debris in the water."