By BBC News Online's Emma Batha

The poisoning of the river Tisza has been called the worst environmental disaster since the Chernobyl nuclear leak in 1986.

A 40km long flow of toxic cyanide has wiped out the river's entire ecosystem in a matter of days - everything from microbes to otters.

On 30 January, 100,000 cubic metres of contaminated water burst through a dam at a mining works in northern Romania.

It has since travelled 1000km through Hungary and Yugoslavia where it has now entered the Danube. Drinking water supplies in all three countries have been poisoned.

When the cyanide first poured into the Somes river, tests showed concentrations were 700 times the permitted level.

But it is the Tisza, Hungary's second biggest river, which has borne the brunt of the catastrophe.

The statistics are mind-boggling. Environmentalists say around 650 tonnes of dead fish have been retrieved so far and expect the figure to double in the next week.

The drinking supplies of 2.5 million Hungarians are threatened and an estimated 15,000 people in the fishing industry have seen their livelihood vanish.

'Nothing is alive'

Biologists say it will take about five years to restock the fish and 10 to 20 years for river life to return.

But they believe some of the damage is irreversible with several species thought to be gone forever.

Around 62 types of fish and 20 protected species have been affected.

Ironically, the disaster happened as Hungary was in the middle of applying to have part of the Tisza placed under the Ramsar Convention, a treaty designed to preserve wetlands of international importance.

''They can throw away the application now,'' said Jozsef Feiler, of Friends of the Earth in Hungary. ''Everything down to bacteria is dead. There's more life in a sewage channel than this river now. Nothing is alive. Zero.''

Volunteers and fishermen are working around the clock to remove the dead fish to stop the disaster spreading further up the food chain.

Hunters are trying to keep larger animals away, but they have already found dead foxes and otters.

Environmentalists are particularly worried about the threat to five pairs of very rare osprey. One has already died from eating toxic fish and another is sick.

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There are also fears that if the poisoned water filters into the soil it could affect grass, grain and livestock.

But Tom Popper of the of the Regional Environmental Centre, based in Hungary, warned that cyanide was not the only danger.

Heavy metals were probably also washed into the river. These take much longer to disappear and could enter the water table.

''In terms of complete destruction to one ecosystem this is probably worse than Chernobyl. The damage to nature has been horrible and the threat to humans could persist,'' Mr Popper added.

The pollution has now reached Yugoslavia, which is already under severe ecological strain following last year's Nato strikes on oil refineries.

About 80% of life in the Serbian section of the Tisza has reportedly been killed off.

The cyanide entered the Danube at the weekend, but scientists say it has now been diluted to within safe limits.

Mourning

Hungarians have been devastated by the catastrophe.

They fondly call the Tisza ''the blonde river'' because of its sandy appearance, and consider it cleaner and prettier than its more famous sister the Danube into which it flows.

Mr Feiler said people had been in tears as they cleared up.

''It's just terrible seeing these huge fish in agony,'' he added. ''It's so shocking seeing this mass extinction on the top of the water.

''We use this river to fish and swim and now it's a danger in the middle of our towns. People are very emotional about it. We've never had anything like this.''

When a person drowns in Hungary, the locals throw flowers into the water and light candles along the banks. This week it is the Tisza itself they have been mourning.

A black flag has been raised at the city hall in Szeged where residents have held candle-lit vigils and dropped flowers off the bridges.

The scenes have been similar in Szolnok to the north, where the mayor Ferenc Szalay has estimated the cost of the disaster will run into billions of dollars.

Hungary and Serbia are both demanding compensation.

'Madness'

The accident happened when a reservoir used for processing gold overflowed a dam at the Aural gold and silver mine, in Baia Mare.

Australia's Esmerelda Exploration, which runs Aural as a joint venture with the Romanian Government, has blamed the flooding on excessive snow falls.

Esmerelda, which owns 50% of the mine, was suspended from trading on the Australian share market last week.

But Esmerelda's chairman Brett Montgomery says the reports of damage have been ''grossly exaggerated''.

The mine operators have suggested the fish could be dying from depleted oxygen levels which can happen when rivers freeze over or flood.

But Hungarian officials say it is ''madness'' to store cyanide so close to a river and that the weather, while unusual, was not unprecedented.

''We know enough about fish to know they're not dying from the cold,'' Mr Feiler snapped.

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