ROME — 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 sludge before it contaminated Italy’s longest and most important river.

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

The cisterns ”were opened by someone who was familiar with the plant and knew how to operate them,” said Cinzia Secchi, a spokeswoman for the Milan provincial government.

There were varying accounts of the amount of oil released: Secchi said officials now believed 2.5 million liters had poured out, down from the initial estimates of 10 million liters but significantly more than the 600,000 liters reported by the ANSA news agency and environmental groups.

Environmentalists warned that several water and bird species were at risk from the spill. 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, and the Po is used extensively for irrigation, the World Wildlife Fund for Nature noted.

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 Turin and Ferrara before emptying into the Adriatic Sea.

The 80-mile Lambro had been polluted by years of industrial runoff well before the spill.