ROME--Tons of oil reached the Po river on Wednesday after a sabotage at a former refinery triggered a large spill into a tributary of Italy's longest river, local officials said.

"The prefect called on local municipalities not to use the water," Roberta De Francesco, head of the prefecture cabinet in the northern city of Piacenza told AFP.

Local authorities are readying barriers to place in the river northeast of Piacenza in order to block and absorb the spill, which is of about 600,000 liters (158,503 gallons), De Francesco said.

The spill began at around 4:00 am (0300 GMT) Tuesday after someone broke into the depot of the former Lombardi Petroli refinery in Villasanta, near Monza, and opened the valves, according to the ANSA news agency.

The spill moved along the Lambro river early on Wednesday to reach the Po river, whose basin covers about a fourth of Italy's territory. Hundreds of birds have already died because of the spill.

"The oil spill came from Monza, passed Milan and is currently in the area of Lodi," Monia Maccarini, a spokeswoman for Lombardy's region environmental protection agency told AFP earlier.

Several attempts to stop the oil moving downstream failed.

Maccarini said the handling of spill was entrusted to Italy's civil protection agency, while Milan's prefecture set up a crisis unit.

Legambiente, Italy's largest environmental organization, called the spill "an ecological disaster without precedent for the Lambro ecosystem".

Running for 652 kilometres (405 miles), the Po flows west to east across the width of northern Italy, wetting the Po Valley, Italy's largest and most fertile plain, before it reaches the Adriatic Sea.