Ednan Tofik ogly Agaev, a former Russian ambassador in Colombia, is charged with international corruption in a case revolving around the 2011 purchase by oil majors Eni and Shell of Nigeria’s OPL 245 oilfield.
Milan prosecutors allege bribes totalling around $1.1 billion were paid to win the licence to explore the field.
Agaev and all the parties involved in the case have denied any wrongdoing.
In documents filed by prosecutors with the Milan court on Wednesday, it emerged that Russia's Foreign Minister Sergei Lavrov handed over a letter to his Italian counterpart in Moscow at a meeting on Oct. 8, calling on Italian authorities to be "reasonable" in the case.
The letter said Russia was convinced Agaev had committed no wrongdoing.
"We hope the Italian authorities adopt a reasonable approach and after respective checks find a way to change the status of E.T. Agaev from accused to witness," the letter said.
The Italian and Russian foreign ministries did not respond to requests for comment.
During the hearing on Wednesday Italian prosecutor Sergio Spadaro described the request as "at the very least surprising". (Reporting by Emilio Parodi, writing by Stephen Jewkes, editing by Susan Fenton)
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