Italian prosecutors have begun an investigation after claims saying Matteo Salvini’s far-right party, the League, sought funds via a Russian oil deal.

Prosecutors in Milan have opened an inquiry into possible charges of international corruption in the case. The inquiry follows an investigation by L’Espresso magazine (reported by the Guardian in February) which alleged there was an arrangement by Russian representatives, close to Vladmir Putin’s government, to sell 3m tons of diesel fuel to an Italian oil company.

The inquiry was launched by magistrates in February but its existence was announced only on Thursday.

Milan’s chief prosecutor, Francesco Greco, told ANSA news agency: “We are carrying out investigations to understand if there are crimes or not.”

According to L’Espresso, on 18 October 2018 a member of Salvini’s entourage, Gianluca Savoini, met two Italians and three Russians in Moscow’s hotel Metropol to discuss a deal that would divert profits to the League before the European electoral campaign in May.

On Wednesday, BuzzFeed News published an audio recording which it said was of that meeting “in which a close aide of [Salvini] and the other five men can be heard negotiating the terms of a deal to covertly channel tens of millions of dollars of Russian oil money to Salvini’s party, the League”.

It alleged that such a deal would represent “a breach of Italian electoral law, which bans political parties from accepting large foreign donations’’.

Reached by the Guardian in February, Savoini, described as the League’s emissary in Moscow, in an email declined to comment on the “fake news”.

The League has denied the reports in L’Espresso and Buzzfeed, and on Wednesday Salvini threatened legal action. “I have already sued for libel in the past, I’ll do it again today, tomorrow and the day after tomorrow,’’ he said.

Although L’Espresso admitted it had no evidence that the deal was ever executed, the claims raised questions about the relationship between the Kremlin and the anti-immigrant party, which is in a coalition government with the anti-establishment Five Star Movement.

Giovanni Tizian, a journalist from L’Espresso, said: “We have documented that the meeting between Savoini and the Russians at the Metropole hotel in Moscow is just the outcome of a series of meetings held in June and July 2018 where Savoini was pushing for a deal with the Russians.” Tizian, along with the investigative journalist Stefano Vergine, co-authored a book entitled The Black Book of the League.

Opposition parties described the claims as worrying and urged immediate explanation. “Rubles from Russia to the League? Salvini has to immediately clarify to parliament,” said Nicola Zingaretti, the centre-left Democratic party’s leader.