A massive oil slick released from a depot in Italy reached the country's longest river, threatening devastation to wildlife.

More than half a million gallons flooded out from an oil depot near Monza after tank valves were deliberately opened, possibly by disgruntled ex-workers.

It has now reached the River Po which crosses the country from Piedmont in the west, across Turin and Ferrara before emptying into the Adriatic south of Venice.

Environmentalists warned that several water and bird species were at risk from the spill as the area is rich in wildlife.

But even after the spill is cleaned up the impact will last as the river valley is the most important agricultural region in Italy, and the Po is used extensively for irrigation, the World Wildlife Fund for Nature said.

The spill began on Wednesday and spread south down the Lambro River to Piacenza and Cremona overnight, despite efforts to contain it.

The 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, noting that Legambiente - as well as the regional government - had asked a state of emergency be declared to free government funds to help contain it.

"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. It's an act of hatred that will be punished by everyone," he said.

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