Fuel was spilled into the Lambro river early Tuesday after saboteurs allegedly broke into the depot of the former Lombardi Petroli refinery in Villasanta and opened the valves, according to media reports.
"We have asked for a state of emergency," President of Lombardy region Roberto Formigoni told reporters Wednesday.
Formigoni said saboteurs "decided to act in a criminal and cowardly way, putting everyone's heritage at risk".
Political parties in the country have called for urgent government intervention to curb the environmental disaster.
An environmental group said Wednesday that the leaked oil has reached the Po River which supplies water to Italy's most fertile agricultural area.
Meanwhile, a task force of firefighters, environmentalists and emergency workers has been set up to control the spread of fuel in the water.
