Deutsche Bank has disappointed customers with problems at its Postbank unit and will work quickly to regain trust, the lender's chief executive Christian Sewing said on Wednesday.

The mea culpa from Germany's top banker comes after Germany's financial regulator admonished the bank for disruptions in Postbank's online offerings, difficulty in reaching customer service and long processing times as "unacceptable".

Deutsche, Germany's largest bank, began acquiring Postbank, with its millions of clients and roots in the country's postal system, in 2008 during the global financial crisis, but for years struggled to complete its integration.

Deutsche said in July it had completed a final phase of the integration, but two weeks ago, the German financial regulator BaFin in an unusual rebuke said it had seen "considerable disturbances" at Postbank.

Sewing, speaking at a financial conference in Frankfurt, said that customers had been disappointed "very much".

"We have not lived up to our responsibility here – and now we have to work all the harder to fix the problems quickly and completely - and regain trust," Sewing said. (Reporting by Tom Sims, Editing by Friederike Heine)