The oil ended up in a containment barrier and officials are checking whether it has leaked into the river, said the Secretariat for Environmental Protection of Vojvodina, the province in which the spill occurred.

The incident happened on April 21 at the Pancevo Oil Refinery, when oil was being transferred from the refinery to a barge owned by Naftachem, a petroleum products company based in Novi Sad.

Local authorities have indicated that, based on initial findings, the spill was most likely to have been the result of negligence by the barge crew.

"It has been established that 82 cubic metres of Euro diesel were spilled and that the fuel was contained within a protective barrier surrounding the barge," the environmental protection institute said.

The Public Prosecutor's Office has launched an investigation into the extent of pollution to the Danube and who may be held responsible.

"Technicians collected samples from seven locations identified by the inspector. The analysis is ongoing and we'll see what the results show," Ljiljana Lazic, director of the Institute of Public Health in Pancevo, told AFP.

Serbia already struggles with water pollution and, according to official estimates, treats less than 15 percent of its wastewater.

The rest, along with sewage waste, is discharged untreated into its rivers.