FRANKFURT — The European Central Bank warned on Wednesday that the euro zone’s slumping economy and a surge in problem loans were raising the risk of a renewed banking crisis, even as overall stress in the region’s financial markets had receded.

In a sober assessment of the state of the zone’s financial system, the E.C.B. said that a prolonged recession had made it harder for many borrowers to repay their loans, burdening banks that had still not finished repairing the damage caused by the 2008 financial crisis.

Last year “was not a good year for banks at all,” Vítor Constâncio, the vice president of the E.C.B., said Wednesday.

While the E.C.B., as customary, did not mention specific banks, it said the most vulnerable were those in countries with high unemployment or falling house prices. That list would include Italy, Spain, Greece and Portugal among others. But ailing banks are also a problem in stronger countries like Germany, where Commerzbank and publicly owned landesbanks, or state banks, are struggling with bad loans to the shipping industry and other problems.