Citigroup Inc posted a first-quarter loss on Friday, reflecting a large amount of credit losses and the accounting for preferred stock.

The bank also said it planned to delay the proposed exchange of billions of dollars of preferred shares into common stock until the U.S. government completes its stress tests of large banks to gauge which might need more capital or aid.

Shares of Citigroup jumped more than 15 percent to $4.63 in trading before the market opened.

Citigroup's quarterly loss available to common shareholders was $966 million, or 18 cents per share, compared with a loss of $5.19 billion, or $1.03 a share, a year earlier. Revenue roughly doubled to $24.79 billion.

Analysts on average expected a loss of 30 cents per share on revenue of $21.73 billion, according to Reuters Estimates.

Excluding the impact of preferred stock, Citigroup said profit from continuing operations was $1.61 billion, vs. a $5.25 billion year-earlier loss.

Results included $10.3 billion of credit costs, up 76 percent, with a large portion of the increase stemming from credit cards. This included $7.3 billion of net credit losses, a $2.7 billion increase to loss reserves, and $332 million for other benefits and claims.

Citigroup has been propped up three times by the government since October, taking $45 billion from the Troubled Asset Relief Program and getting a government agreement to share in losses on $300.8 billion of troubled assets.

At Thursday's close, the company's shares had fallen 40 percent this year, compared with a 19 percent drop in the KBW Bank Index <.BKX>. Citigroup had traded above $56 as recently as January 2007.

(Reporting by Jonathan Stempel)