This was confirmed by the office of the Swiss Attorney-General saying the action was taken at the request of public prosecutors from Milan.
Three separate bank accounts in Lugano, Basel and Geneva were blocked, according to media reports confirmed by prosecutors but they declined to reveal the value of the seized assets.
It revealed that it has also confiscated, assets and shared information with Italian authorities in view of the pending trial over the oil deal.
Nigerian and Italian investigators accuse officials of both Eni and Shell of shady financial deals to secure a licence for the lucrative OPL 245.
The trial of the executives, including former Eni’s Claudio Descalzi, and his predecessor, Paolo Scaroni, was originally slated for March but was moved to May after it was transferred to Milan.
In all, apart from Messrs Descalzi and Scaroni, 11 other people including two former top Shell managers, former Nigerian oil minister, Dan Etete and a series of middlemen and advisers are being probed.
Eni and Shell were also listed as corporate defendants in the case. Both oil companies have denied wrongdoing and expressed confidence that the trial would exonerate both the companies and individuals.
Apart from Mr Etete, a former Attorney-General, Bello Adoke, are amongst several Nigerians indicted in the deal, which was approved by ex-President Goodluck Jonathan, who also denies wrongdoing.