Citigroup has reported a massive jump in quarterly profit as a sharp drop in legal costs and gains from the disposal of unwanted assets more than made up for weak revenue from its core business. 

The bank's chief executive Michael Corbat has been looking to shrink Citi to a more profitable core, focus on more promising markets and return capital to shareholders.

The bank's legal and repositioning costs plunged to $724m in the fourth quarter from $3.55 billion a year earlier. 

Adjusted revenue from its main Citicorp business declined 2%, but profit rose as expenses fell 24% in the unit. 

"We have undoubtedly become a simpler, smaller, safer and stronger institution," Corbat said in a statement.

"We have sharpened our focus on target clients, shedding over 20 consumer and institutional businesses in the process," he said. 

Investors, however, have been skeptical of Corbat's story and are worried that the bank's results will be undermined by slowing growth in emerging markets, where Citi has more assets than other US banks. 

Citi said its net profit rose to $3.34 billion, or $1.02 per share, in the quarter ended December 31 from $344 million, or 6 cents per share, a year earlier.

Excluding items, the bank earned $1.06 per share, beating the average analyst estimate of $1.05, according to Thomson Reuters. 

The bank's adjusted revenue rose 4.2% to $18.64 billion, but the increase came from gains on disposal of assets from Citi Holdings, which shrank 43% from a year earlier. 

Total expenses fell 22.8% to $11.13 billion.

The lender set aside about $250m to cover losses related to its energy portfolio as depressed crude prices pressure US banks' loans to oil companies.