US sanctions on Russian-owned NIS have prevented the Serbian oil group from receiving a crude cargo that could have bought time for Serbia's sole refinery, which faces closure without new supplies, sources with knowledge of the matter said.
The sources, who spoke on condition of anonymity due to the sensitivity of the situation, told Reuters that Serbia has just days before the NIS oil refinery at Pancevo will be forced to stop processing crude.
NIS did not respond to Reuters' requests for comment.
Serbia's President Aleksandar Vucic warned on October 9 that without deliveries, the refinery, which supplies most of the country's oil products, including gasoline and jet fuel, would struggle to operate beyond November 1.
However, Vucic said that fuel storages are full and existing stocks of oil products should keep Serbia supplied until year end. "There will be no shortages of crude oil, its derivatives and no energy crisis," he told Serbs last week.
Russia and Serbia are still working to find a solution that would lift the US sanctions, announced in January to target NIS' majority Russian ownership, which came into effect on October 9 after a final waiver expired.
Vucic is facing a wave of anti-government demonstrations and may now have to try to secure extra barges of emergency oil when cold winter weather hits the Balkan country.
The tanker Maran Helios carrying one million barrels of Kazakh KEBCO crude bound for NIS in Serbia arrived at Croatia's Omisalj on October 9, data from analytics group Kpler shows.
But the fuel did not then make it to Serbia, a source close to the matter said.
Croatian pipeline operator Janaf, which had been given an extension to continue transporting oil to Serbia until October 15, said in a statement this week that it had delivered everything in its system owned by NIS on October 8.
Janaf said it had no further oil due for delivery to Serbia after this date, suggesting it had not taken receipt of the Kazakh KEBCO crude cargo which NIS had bought.
Reuters could not determine if the cargo will be stored at Omisalj or resold to a different buyer, after it finally discharged on Tuesday after hovering off the port for about two weeks, Kpler data showed.
The crude which NIS was due to have received would have been enough to run the Pancevo refinery for around 10 days, according to Reuters calculations.
While two sources told Reuters that Serbia's fuel stocks are almost full, the country will become increasingly reliant on imports as they start to run down.
"For me it's not even a question. The refinery must work, and petrol stations must stay operational," Nadezda Kokotovic, former head of NIS' EU liaison office, told Reuters.
(Reporting by Robert Harvey in London, Ivana Sekularac and Aleksandar Vasovic in Belgrade, and Daria Sito-Sucic in Sarajevo. Editing by Edward McAllister, Alex Lawler and Alexander Smith)