Russia's largest lender, Sberbank, reported a deterioration in the quality of its loan book in the second quarter as the share of impaired loans increased and executives warned that corporate borrowers face mounting pressure from slowing economic growth, high interest rates and a strong ruble.

The share of impaired loans in Sberbank's portfolio rose to 5.5% at the end of the second quarter from 4.8% three months earlier, according to the state-controlled lender's financial statements.

Sberbank attributed the increase primarily to a gradual deterioration in its corporate loan portfolio, while noting that it had also written off fewer retail loans than in previous quarters.

The bank's total loan portfolio stood at 52.6 trillion rubles ($657.5 billion), implying impaired loans of about 2.9 trillion rubles ($36.3 billion).

Overdue loans totaled 2.6 trillion rubles ($32.5 billion), according to the bank's financial statements. Of those, loans worth 1.6 trillion rubles ($20.0 billion) were more than 90 days past due, including 636 billion rubles ($8.0 billion) in corporate loans, 144 billion rubles ($1.8 billion) in project finance, 158 billion rubles ($2.0 billion) in mortgages and 563 billion rubles ($7.0 billion) in consumer loans. Restructured loans are not classified as overdue under Sberbank's reporting.