German authorities will introduce financial rules this month that could delay or even block an attempt by the liquidity-strapped Iranian regime to claim cash parked in Germany that has become an irritant in the relationship between Washington and Berlin.

U.S. officials said new anti-money-laundering rules being drafted by the German central bank could complicate and perhaps stop Iran from shipping €300 million in cash currently held by an Iranian-controlled bank in Hamburg. The central bank confirmed the new rules without...