Four bankers from a Russian bank's Swiss branch have been found guilty by a Zurich court over vast sums going into the accounts of a close confidant of President Vladimir Putin.

The accounts in the Swiss arm of Gazprombank were held by Sergei Roldugin, the artistic director of the St. Petersburg Music House, who is often dubbed Putin's cellist.

Roldugin has been a friend of Putin for more than four decades and is godfather to one of the Russian leader's daughters.

The four men were found guilty of "lack of due diligence in financial transactions", the Zurich District Court said in its verdict released to media on Thursday, over the millions of Swiss francs flowing through Roldugin's account.

Under Swiss law, the bankers -- two Russians, one Swiss and a Russian-born British national -- cannot be identified.

The bank branch's chief executive was fined 540,000 Swiss francs ($590,200).

Two other managers were fined 90,000 francs and 63,000 francs, while a customer advisor was fined 48,000 francs.

But the sentences were suspended for two years, meaning the four will not have to pay the fines unless they reoffend within that time.

The court confirmed that the four men had exercised their right to appeal against the verdict.

Swiss law requires banks to apply more stringent rules for transactions conducted by people who exercise high public functions abroad or are deemed to be close to power through their family, personal or business relationships.

The Zurich cantonal public prosecutors accused the Gazprombank four of not having taken the necessary precautions to ensure that Roldugin was the legitimate beneficiary of the sums entrusted to the bank between 2014 and 2016.

The cellist and conductor was sanctioned by the European Union in the days following Russia's February 2022 invasion of Ukraine, with the European authorities saying he was "part of Putin's hidden financial network".

And in June, the US Treasury issued sanctions, saying he was a key figure in managing Putin's personal wealth offshore.

Gazprombank is the financial arm of Russian natural gas giant Gazprom. Its Swiss branch focused mainly on products and services intended to finance international trade operations.

It was banned from the Swiss banking sector following the invasion of Ukraine but is still in the process of liquidating its business in the Alpine country.