Commerzbank Chief Financial Officer Stephan Engels said the default of Detroit had dented second-quarter earnings, adding that the lender had made "adequate" provisions.
"It is a reasonable, substantial number that has been taken into impairment," Engels said about the scale of the Detroit hit, declining to put a figure on it.
Separately, the Frankfurt-based bank said business with small to medium-sized companies would be lower in the third quarter when compared with the second quarter.
The Frankfurt-based bank said provisions for bad ship loans in 2014 will most likely be on the same level as 2013, despite the fact that some market participants are making positive noises.
Shares in the bank soared over 10 percent in morning trade.
Net profit of 43 million euros ($57.27 million) at Germany's number two lender missed the 65 million average forecast in a Reuters poll, as strength in the investment bank failed to offset weak results at the Mittelstandsbank.
"The figures are broadly in line with the low expectations, with two exceptions: significantly better earnings in the investment bank and worse results in the Mittelstandsbank", said LBBW analyst Ingo Frommen.
(Read more: Societe Generale profit more than doubles)
The bank declined to give a specific 2013 outlook, reiterating that 2013 will be a year of transition.
"As a consequence of the portfolio reduction and the ongoing weak interest rate environment, the revenues before loan loss provisions are likely to remain under pressure," Chief Financial Officer Stephan Engels said in a statement.
Commerzbank's cash cow Mittelstandsbank unit, which specializes in providing loans to Germany's important medium-sized companies, posted a fall in operating profit to 216 million euros in the quarter, down from 386 million in the year-earlier period.
(Read more: BNP Paribas profit falls, expands in Germany)
While earnings of the retail banking division were slightly up, the loss at the non-core asset division widened, as Commerzbank wrote down its public finance exposure - including bonds of bankrupted U.S. city Detroit.
A spike in earnings at the investment banking unit prevented the lender from sliding into the red.
By contrast, second-quarter earnings at French peer Societe Generale doubled as securities trading and foreign retail banking surged, while BNP Paribas and Italy's Unicredit also beat analysts' expectations.