Efforts have been underway using at least five floating barriers to trap the oil on the waterway's surface. But it is proving to be a difficult task for civil protection officials and fire-fighting teams dispatched to the area.

Two Italian regions, Lombardy and Emilia Romagna, have already requested that a state of emergency be declared. Environment Minister Stefania Prestigiacomo flew over the entire affected area by helicopter on Thursday.

The oil spill began Tuesday following what environmental authorities are calling "a true act of environmental terrorism." The oil stored in tanks in a former refinery was sabotaged when someone opened the main valve.

Police say the saboteur must have had a working familiarity with the refinery to be able to open the tank's main valve and send tons of oil pouring out.

Officials say the act was intentional and an investigation has been opened into the spill, which is already more than 100 kilometers long.

Damiano di Simine, Lombardy president for the Legambiente environmental group, said immediate action should have been taken to avoid such a disaster.

The criminal act itself is extremely serious, he says, but even more serious is the fact that the oil was allowed to pour out for hours without anyone intervening or even realizing what was happening.

Meanwhile the oil spill has reached Italy's important food-producing province of Parma. But farm groups were quick to reassure that the food chain is safe because farm production is low at this time of the year.