A giant oil slick threatened wildlife, fishing grounds and tourist spots Thursday as it moved down the Po, Italy’s longest river, towards the sea, defying all efforts to stem its progress.

Environment Minister Stefania Prestigiacomo called the spill, blamed on saboteurs who broke into a disused refinery and opened valves, “a true attack on the environment and on citizens’ health,” the ANSA news agency reported.

Several kilometers (miles) long, the slick was halfway between the cities of Cremona and Mantua late Thursday, having covered about 200 kilometres (125 miles) since it was released into a tributary of the Po early Tuesday.