ZURICH (Reuters) - Swiss bank UBS on Sunday increased the amount it said it had lost on rogue equity trades to $2.3 billion and alleged a trader concealed his risky deals by creating fictitious hedging positions in internal systems. UBS stunned markets on Thursday when it announced unauthorised trades had lost it some $2 billion. London trader Kweku Adoboli was charged on Friday with fraud and false accounting dating back to 2008. The loss resulted from unauthorised speculative trading in various S&P 500, DAX, and EuroStoxx index futures over the last three months, UBS said in a brief statement. The loss arising from this matter is $2.3 billion. As previously stated, no client positions were affected. Global stock markets have been extremely volatile in recent months, plunging on concerns over euro zone and U.S. debt crises and then rebounding on hopes for their resolution. The alleged fraud is a disaster for the reputation of Switzerlands biggest bank, which had just started to recover after it almost collapsed during the financial crisis and faced a damaging U.S. investigation into aiding wealthy Americans to dodge taxes. Loss even more. Reads like theyre making excuses, said Helvea analyst Peter Thorne of the UBS statement. The new scandal has prompted calls for its top managers to step down and for its investment bank to be split into a separate unit from its core wealth management business. Chief Executive Oswald Gruebel, who was brought out of retirement in 2009 to turn the bank around, was quoted on Sunday as saying he is not considering quitting over the crisis, but said it was up to the board to decide. If you ask me whether I feel guilty, I say no, he told Der Sonntag newspaper. If somebody proceeds with criminal energy, we cannot do anything. Swiss newspapers quoted unnamed insiders as saying the UBS board and important shareholders such as the Singapore sovereign wealth fund were still backing Gruebel, with immediate changes at the top the last thing the bank needed. Gruebel is widely expected to present plans to drastically cut back the investment bank at an investor day in November. The bank, whose three keys logo symbolise confidence, security, discretion, has pulled its We will not rest global advertising campaign for now, that was designed by advertising agency Publicis to try to rebuild its image. Meanwhile, UBS client advisers have been writing to customers to reassure them of the underlying financial strength of the bank despite the trading loss, a spokesman said. UBS said its board of directors had set up a committee chaired by independent director David Sidwell, former chief financial officer at Morgan Stanley, to conduct an independent investigation into the trades and the banks control systems. The bank said it had covered the risk resulting from the unauthorised trades, and its equities business was again operating normally within previously defined risk limits. It said the trader had allegedly concealed the fact his trades violated UBS risk limits by executing fake exchange-traded fund (ETFs) positions. Following inquiries directed to him by UBS control functions that were reviewing his positions, the trader revealed his unauthorised activity, the bank said in a statement. The positions taken were within the normal business flow of a large global equity trading house as part of a properly hedged portfolio, UBS said. However, the true magnitude of the risk exposure was distorted because the positions had been offset in our systems with fictitious, forward-settling, cash ETF positions. ETFs are index funds listed on an exchange and can be traded just like regular stocks. They try to replicate index performances and offer lower costs than actively managed funds, but regulators have warned about risks from some complex ETFs. In the past three months, DAX futures have fallen 22 percent, Eurostoxx 50 futures have dropped 20 percent and S&P 500 futures have dipped 4 percent. The instruments involved in the UBS case are similar to those that Jerome Kerviel, the rogue trader at Societe Generale, traded when he racked up a $6.7 billion loss in unauthorised deals in 2008. The Sunday Times cited unnamed insiders saying the trader placed bets worth $10 billion before his losses were detected. Christoph Blocher, vice-president of the right-wing Swiss Peoples Party (SVP) -- the countrys biggestrenewed his calls for a splitting off of the investment bank. One has to seriously examine a ban on investment banking for commercial banks, he told the SonntagsZeitung, adding his party might team up with the center-left Social Democrats to push for such a move.