LUCERNE, SWITZERLAND - Switzerland's biggest bank UBS has uncovered a number of tax fraud cases, its chairman said on Thursday, insisting that banking secrecy laws were not devised to shield tax cheats.

"Our investigations have uncovered a limited number of cases of tax fraud under both US and Swiss law," Peter Kurer told over 2,000 shareholders during an extraordinary general meeting here.

"Bank secrecy... is not there to protect cases of tax fraud," he added, defending the bank against accusations that it had violated bank-client confidentiality.

UBS is facing a US Justice Department investigation into whether it had helped American clients evade taxes.

In July, it said that it was halting its offshore banking services for US citizens.

The bank is also struggling to restore investor and client confidence after losing billions in the US subprime home-loan crisis, which forced the bank to turn to the state for a rescue package worth almost 60 billion dollars.

In the aftermath of the steep losses, clients withdrew a massive 83.7 billion Swiss francs in the third quarter.

This week, the bank unveiled a new advertising campaign with the tagline "Everyone deserves a second chance. UBS too."

Kurer sought to calm nerves at Thursday's meeting, admitting that the bank could communicate better with investors.

"It is true that we should have been even more transparent," said Kurer. He also addressed another cause anger for investors - senior executives' bonuses which ran into millions of francs.

Former chairman Marcel Ospel has come in for particular criticism, with tabloid newspaper Blick running a campaign to get him to repay his bonuses to the bank.

On Tuesday, Ospel together with former board members Stephan Haeringer and Marco Suter returned 33 million francs to the bank.

Earlier this month, Peter Wuffli, the former head of UBS's executive board, also waived 12 million francs to which he was entitled.

Kurer revealed that in all, former senior executives have voluntarily given up about 70 million Swiss francs in bonuses and wages, including 22 million foregone by other former senior executives who were not named.

"A total of approximately 70 million francs (58.28 million dollars, 45.2 million euros) in bonuses has been repaid by executives who have left the bank," Kurer said.

"Once again, UBS is a leader in this regard, as no other bank has seen so many former top managers agree to forego so much," he added.