An oil spill that fouled a small river in northern Italy reached the Po River on Wednesday, with officials warning of an ecological disaster as they scrambled to contain the spill before it contaminated Italy's longest and most important river.

Milan regional officials said the cause of the spill was likely sabotage at a former refinery turned oil depot, since the cisterns were apparently opened and allowed to flow into the Lambro River near Monza.

Environmental groups and the ANSA news agency estimated the amount of oil at 600,000 litres, down from initial estimates of some 10 million litres.

The spill began Tuesday and spread south down the Lambro to Piacenza and Cremona overnight, despite efforts to contain it. By Wednesday, it had reached the Po, which crosses the country from Piedmont in the west, across Milan and Verona before emptying into the Adriatic sea.

Environmentalists warned that several water and bird species were at risk from the spill, since the area is rich in bird and other wildlife. But even after the spill is cleaned up the impact will last as the Po river valley is the most important agricultural region in Italy, with the Po used extensively for irrigation, the World Wildlife Fund for Nature noted.

The 130-kilometre Lambro — a tributary to the Po that means "clear" in Latin — had been polluted by years of industrial runoff well before the spill.

But Damiano di Simine, regional president of the Legambiente environmental group, said the slick had caused even more damage to a tributary that had just recently shown signs of recovery, with fish returning.

"The scale of this is dramatic," he said in a phone interview, noting that Legambiente — as well as the regional government — had asked that a state of emergency be declared to free up federal funds to help contain it.

Oil-covered ducks

"We don't yet know the details, but there is great damage to the ecological system — all the vegetation and fauna," he said.

Several oil-covered ducks have already been plucked from the river and taken for treatment at a regional animal shelter.

The WWF said most at risk were fish, wild ducks and herons, who in these days were beginning to nest along the Po.

The Lombard regional president, Roberto Formigoni, said those responsible would be prosecuted and punished severely for what his office called an "ecological disaster."

"Some criminal decided to intervene in a harmful and cowardly way, putting at risk an asset that belongs to all of us," the Apcom news agency quoted Formigoni as saying. "It's an act of hatred that will be punished by everyone."

While no arrests have been made, Italian news reports have noted that the depot owner, Lombarda Petroli, had laid off several workers in recent months as it downsized.

Di Simine charged that the company had in the past year managed to get off a list of at-risk industrial plants, which would have required it to keep up safety standards and an emergency contingency plan to deal with a spill.

There was no answer at the company Wednesday.