At least 4,500 birds are known to have died at Lake Koronia, a nature reserve supposedly protected from industrial pollution, for unknown reasons in just over a week, and the toll could rise to 15,000 or more, Xenofon Kappas said.

Twenty-nine species of birds, including several protected ones, have been hit, and the dead include some 150 pelicans as well as gulls and ducks, at the lake just 10 kilometres (six miles) east of the northern city of Salonika.

According to Salonika deputy prefect Yiannis Bikos, an as yet unidentifiable substance paralyses the bird's nerve system after they enter the water.

"They birds either drown right away or they die after crawling to the shore," Bikos said.

Volunteers, hunters and ecologists are gathering the bodies of the dead birds -- many of them in areas difficult to reach -- and burning them on huge pyres.

Grazing in the area was banned. Visitors, particularly children, are urged to stay away "as a protective measure" until the cause of the deaths is determined.

The outbreak of deaths was first noted just over a week ago. But authorities have managed to slash the number of new deaths in the past few days, the official said. Local authorities have installed some 40 little cannons around the lake to scare away arriving birds with their bang. Most fly to the larger, nearby lake Volvi.

But the crisis took an even nastier turn on Friday when the lake's fish, which had been hitherto spared, began to die in large numbers. Around 1,000 fish of different kind and size were seen floating dead on Koronia's surface.

"Volunteers saw them in a corner of the lake where the first dead birds had been spotted," Bikos said. Fishing had been banned in the lake as soon as the first dead birds were discovered.

A toxic analysis of the waters is currently under way at Salonika university. Samples will be sent to Britain on Monday. According to Bikos, initial results point to clostridium botulinum, the spore-forming rod that produces the potent neurotoxin that causes botulism.

Clostridium botulinum develops in stagnant or polluted waters, with wild water fowl and some fish known to be affected. Foodbourne ingestion of the neurotoxin has a high mortality rate for humans if not treated immediately.

Experts also established an unusual proliferation of microalgae, a sort of seaweed, in the lake.

Toxic waste by manufacturing plants around Koronia is not to blame for the incident, Bikos said.

But the agent that killed the birds could have never developed its full lethal power had authorities not allowed the draining of the lake for farming purposes, according to Kappas.

Lake Koronia has lost a third of its surface in the last 30 years, shrinking to 3,000 hectares (7,400 acres).

"The level of the lake has gone down. The concentration of oxygen in the water is low," Bikos said.

Lake Koronia is formally protected under the EU-sponsored Natura 2000 network and the international Ramsar environment protection treaty.

Dozens of birds on the same lake were killed back in 1995, said Dimitris Papadimoulis, a Greek member of the European Parliament, who urged the bloc's executive Commission to intervene in the matter.

The European Commission has repeatedly sued Greece for violation of EU environmental laws.

"It seems that plenty of European Union funds to protect the site have been wasted," Kappas said.

"If we fail to protect our natural treasures, we could be fined huge sums under the treaties," Bikos said.