The letter, seen by Reuters, is signed by the head of the Office of Foreign Assets Control (OFAC) at the U.S. Treasury, and mentions a loan provided by some Italian banks and state lender Cassa Depositi e Prestiti (CDP).
The Italian government adopted on Thursday a scheme allowing ISAB to be placed under trusteeship, while Lukoil continues talks on selling the asset.
A similar move was taken by Germany when in September it took control of a refinery owned by Rosneft (ROSN.MM).
The government could call on "an oil company that operates in the sector, and it is obvious to everyone that this (company) could be Eni, and this will ensure continuity of production," he said.
Reporting by Giuseppe Fonte and Alvise Armellini, Editing by Christina Fincher and Louise HeavensOur Standards: The Thomson Reuters Trust Principles.