Swiss bank UBS warned on its first-quarter performance when posting quarterly profit at the bottom end of expectations after a loss at its investment bank, amid uncertainty around the euro zone, Europe's banking system, and U.S. deficit issues.

Traditional improvements in first-quarter activity levels and trading volumes may fail to materialize fully, which would weigh on overall results for the coming quarter, most notably in the investment bank, UBS said on Tuesday.

UBS, trying to rebuild confidence after a $2.3 billion rogue trader scandal uncovered in September, said inflows at its flagship private bank should hold up.

We believe our asset-gathering businesses as a whole will continue to attract net new money as our clients recognize our efforts and continue to entrust us with their assets, it said.

The private banking arm's inflows slipped to 3.1 billion Swiss francs ($3.4 billion) in the fourth quarter from 3.8 billion in the previous quarter.

UBS also said it would issue loss-absorbing capital to meet tough new Swiss capital rules.

The bank, which until now has pledged to steer clear of contingent convertible bonds did not elaborate on which form of loss-absorbing capital it would issue.

Fourth-quarter net profit shrank to 393 million Swiss francs from 1.66 billion francs in the 2010 period and compared with a forecast for 737 million in a Reuters poll. [

($1 = 0.9227 Swiss franc)

(Reporting By Katharina Bart; Editing by Dan Lalor)