The daily Corriere Della Sera reported that Eni was being investigated over payment of suspected bribes when oil licences in Congo-Brazzaville were being renewed.
Eni confirmed to AFP that on Thursday the Milan prosecutor's office requested "documents related to certain activities in the Congo in 2009 and 2014".
"The request follows two others received in 2017," its spokesman said, adding that the firm had already handed over the documents and was innocent of any misconduct.
Eni also confirmed that the offices of two Eni officials had been searched.
Italian media reported that Eni agreed to include in its contracts -- worth €350 million -- Congolese companies identified by the country's government, including the Africa Oil and Gas Corporation run by Denis Gokana, oil advisor to Congolese President Denis Sassou Nguesso.
In addition to Eni as a corporation, five individuals are being investigated, including two Eni executives: head of development operations and technology Roberto Casula and director Maria Paduano.
The allegations come just over a month before Eni is set to go on trial for corruption in Nigeria, where Milan prosecutors suspect the payment of bribes during the 2011 purchase of the OPL-245 offshore oilfield.
Former Eni CEO Paolo Scaroni is also facing a possible six years in prison in another ongoing trial regarding bribery in Algeria between 2007 and 2010.