Germany's second biggest-lender, Commerzbank, also said in a regulatory filing that investigations by the United States into violations of sanctions on Iran and other countries could hold "considerably negative" consequences.
Commerzbank, which is 25 percent-owned by the German state, said U.S. authorities were investigating whether its dealings with Iran, Sudan, Myanmar, North Korea and Cuba had violated U.S. embargoes, and pointed out that other banks had paid large settlements to end such investigations.
"The financial impact of the procedure and its termination cannot be predicted and could exceed eventual provisions, which could have considerably negative consequences," Commerzbank said.
Commerzbank repeated on Wednesday that it had had no new business with Iran since 2007 and that it was too early to say what the financial consequences of the U.S. probes would be.
Washington imposed economic sanctions on Tehran in 1979 after Iranian students stormed the U.S. embassy and took diplomats hostage. Until November 2008 U.S. banks could process some transactions for Iranian banks or individuals provided they were initiated offshore by non-Iranian foreign banks and were on the way to other non-Iranian foreign banks.
The European Union has also imposed sanctions, including a ban on trading Iranian oil, due to fears that Tehran is trying to develop nuclear weapons. Iran says the programme is purely peaceful but the measures are making it increasingly difficult for Tehran to conduct business in U.S. dollars and euros.
The United Nations Security Council has also introduced more limited restrictions.
Standard Chartered agreed last week to pay $340 million to the New York bank regulator after it was accused of concealing $250 billion in Iranian transactions. The London-based bank joined a long list of lenders which have been punished for doing business with sanctioned states such as Iran and Cuba.
Barclays Plc, Lloyds Banking Group Plc, Credit Suisse and ING Bank NV have agreed to fines and settlements totaling $1.8 billion, while regulatory filings show that HSBC Holdings Plc is under investigation.
In 2010, RBS agreed to pay $500 million to settle similar allegations by U.S. federal authorities that ABN Amro, a Dutch bank RBS acquired in 2007, had violated U.S. sanction laws.
Analysts expect RBS to settle with U.S. and UK regulators over its involvement in the Libor scandal later this year. It has also endured problems on the domestic front, with a computer systems failure causing massive disruption to customers in June.
Shares in RBS were down 0.4 percent to 236.5 pence at 1120GMT. Commerzbank's shares were down 1.5 percent at 1.27 euros by 1120 GMT. The STOXX Europe 600 banking index was down 0.8 percent.