Even in this troubled banking environment, the problems evident in the quarterly reports of several regional banks are catching investors by surprise.

Second-quarter results released Tuesday by Regions Financial Corp. (RF) and Comerica Corp. (CMA) were the latest to show that regional banks are plagued by big increases in problem loans. Commercial real estate and construction loans are the main concern.

Shares of the two banks were down sharply around midday Tuesday, even as major U.S. stock indexes were little changed. Regions shares were down 14% at $3.47, while shares of Comerica fell 9.9% to $20.56.

Several bankers, carefully mincing words, have expressed optimism lately about certain loan portfolios where delinquencies appear to show early signs of stabilization. But soured loans already on the books will take several quarters to work themselves out and will likely dampen earnings for quarters to come.

Regions reported a $188 million second-quarter loss, compared with a $206 million profit a year earlier. Though its loss of 22 cents a share was in line with analysts' expectations, investor sentiment was soured by news that nonperforming loans, those loans for which collection is doubtful, rose faster than expected.

"Many investors have been concerned that Regions still has a long way to go in terms of loss recognition, and these numbers seemed to verify that," said Sanford C. Bernstein & Co. analyst Kevin J. St. Pierre.

Loan losses at the Birmingham, Ala., bank increased in virtually all categories compared to the first quarter. Losses in bank's two biggest loan portfolios, business and commercial real estate, increased 45% and 6%, respectively.

Fox-Pitt Kelton Cochran Caronia Waller analyst Albert Savastano said that the spillover of souring loans in commercial real estate loans tied to retailers and multi-family properties was particularly concerning.

Regions has been struggling for some time with its real estate exposure to Florida and other Southern markets. Regions needed to raise the most capital relative to risk-weighted assets following the government-induced stress test for the top 19 banks.

On a positive note, the bank said deposits rose and it continued to make new loans.

Comerica, a commercial lender that has so far been somewhat isolated from the worst part of the real estate crisis, also surprised with a bigger than expected increase in soured loans.

The bank is hopeful that losses this quarter will not rise from second-quarter levels, and that the performance will improve in the fourth quarter. The Dallas bank, however, noted during a conference call with investors that loan demand remains timid.

Comerica, which relocated its headquarters to Dallas from Detroit two years ago, reported second-quarter income of $18 million, a 68% decline from a year ago but double the income from the first quarter of this year. Loan losses rose sharply in each comparison.

"Up until this quarter the company has done a better job than most banks we cover in providing accurate guidance on loan losses," Stifel Nicolaus & Co. analyst Christopher Mutascio wrote in a research report. But second-quarter charge-offs "and management's outlook is now much higher than it was just three months ago."

BB&T Corp. (BBT), a bank considered by many analysts to be more solid than most, reported a big jump in nonperforming loans in its quarterly report Friday, as did Zions Bancorp (ZION) on Monday.

BB&T shares were off 3.9% at $20.17 in recent trading, while Zion shares fell 13.3% to $10.59.

Citigroup Global markets analyst Greg Ketron believes Zions' rising loan losses will require about $900 million in fresh capital between 2010-2012, which he said "translates to potential (earnings per share) dilution in the 50% range."

-By Matthias Rieker, Dow Jones Newswires; 212-416-2471; matthias.rieker@dowjones.com