ZURICH (Reuters) - UBS <UBSG.S> Chief Executive Sergio Ermotti said on Thursday political uncertainty and concerns over global growth meant clients remained cautious about the market in the third quarter.

"With respect to what we have experienced so far in the third quarter, normal seasonality, underlying macroeconomic uncertainty and heightened geopolitical tensions continue to contribute to client risk aversion and generally low transaction volumes," Ermotti said at a Bank of America Merrill Lynch conference in London.

"In some businesses and regions in which our IB (investment bank) operates, conditions have remained challenging through the third quarter," Ermotti added. UBS is the world's biggest wealth manager.

(Reporting by Joshua Franklin; Editing by Michael Shields)