Italian government officials are growing frustrated at Germany’s opposition to a potential takeover of Commerzbank AG by Milan-based UniCredit Spa, threatening to disrupt the delicate diplomatic relations between Berlin and Rome.

Germany’s chancellor decried “unfriendly attacks” on banks after UniCredit moved to raise its stake in the lender.

Italy's UniCredit defied Germany's defence of Commerzbank by using derivative contracts on Monday to raise its potential stake in the German bank close to 21%, while waiting for regulatory approval to go above 9.