Citigroup (C) drove a 21% increase in fourth quarter earnings, but that wasn't enough to reach analyst expectations as a weakening mortgage business proved too tough a headwind to overcome.

The nation's 3rd largest lender said it earned $2.6 billlion in the fourth quarter, versus $2.15 billion one year earlier; Q4 earnings represented $0.85 per share, missing analysts' concensus expectation of $0.94 per share.

Shares fell 3.3% in pre-market trading on the earnings news.

"Although we didn't finish the year as strongly as we would have liked, we made substantial progress toward our key priorities in 2013," said Micheal Corbat, chief executive at Citigroup.

"Having grown our operating net income by 15% over 2012, we achieved our highest amount of net income since before the financial crisis," he added.

Citigroup's full year 2013 net income was $13.9 billion on revenues of $76.4 billion, compared to net income of $7.5 billion on revenues of $69.1 billion one year earlier.

Citigroup revenues of $17.9 billion in the fourth quarter were 2% below the prior year period, reflecting lower revenues primarily due to lower mortgage refinancing activity, the bank said.