ZURICH, May 26 (Reuters) - A Zurich court is on Monday hearing an appeal by bankers convicted last year of failing to perform due diligence in financial transactions after helping a close friend of Vladimir Putin move millions of francs through Swiss bank accounts.

The four former employees of the Swiss unit of Russia's Gazprombank, which is currently being wound up, are asking Zurich's High Court to overturn the conviction.

The three Russians and one Swiss citizen helped Sergey Roldugin, a concert cellist who has been dubbed "Putin's wallet" by the Swiss government and is godfather to Putin's eldest daughter, deposit millions of francs in Swiss accounts between 2014 and 2016.

The bankers, who cannot be identified under Swiss reporting restrictions, were found guilty by Zurich's District Court in March 2023 and given suspended fines totalling hundreds of thousands of Swiss francs.

They are appealing the ruling that they did not look closely enough in their dealings with Roldugin.

The presiding judge explained that the case was not about who the money belonged to and whether it came from illegal sources. "The only issue in these proceedings is whether the required checks were made," he said.

In Switzerland, banks are obliged to reject or terminate business relationships if there are doubts about the identity of the contracting party.

The four helped the cellist operate two bank accounts at Gazprombank in Zurich, through which millions of francs flowed, without conducting sufficient checks, the court heard last year.

The indictment said the case showed how people like Roldugin were used as "strawmen" to hide the true owners of money.

Prosecutors alleged the men failed to do enough to determine the identity of the actual owner of the funds and it was implausible that Roldugin, who has no listed activity as a businessman, could be the real owner.

Roldugin was among members of Putin's inner circle sanctioned by the West after Russia invaded Ukraine in 2022.

The Kremlin has previously dismissed any suggestion that Roldugin's funds are linked to the Russian leader as anti-Russian "Putinophobia".

It says Putin's finances are a matter of public record and he has regularly publicly declared his assets and salary.

The three judges are expected to reach and announce a judgment on the appeal towards the end of the day at the earliest.

($1 = 0.9137 Swiss francs) (Reporting by Noele Illien; Editing by Jan Harvey)