Iran is already under UN sanctions for its nuclear activities
The European Union has agreed new financial sanctions aimed at pressuring Iran to halt uranium enrichment.
The new measures freeze all assets of the Bank Melli, Iran's biggest bank, within the EU.
Last week the EU's foreign policy chief Javier Solana offered new incentives to Tehran - but has not received a reply.
Iran insists its nuclear programme is entirely peaceful, but is defying a demand from the UN that it stop the enrichment of uranium.
The UN Security Council approved a third round of sanctions against Iran over the issue in March 2008.
Western officials have accused Bank Melli of providing services to Iran's nuclear and ballistic missile programmes.
The United States placed the bank on its blacklist last year, a measure that allows the freezing of any assets found in the US.
Existing UN Security Council sanctions block the sale to Iran of equipment and technology related to nuclear activities.
They also ban the sale of so-called dual-use items, and call on governments to withdraw financial backing from companies trading with Iran, and prevent dealings with an Iranian state bank and a number of named individuals and organisations.
Earlier this month, Russia, China, the UK, France and Germany, renewed a package of incentives to Iran to try to encourage the opening of negotiations.
Under the package, talks can start about a long-term agreement on Iran's nuclear programme if Iran halts uranium enrichment.
Also on offer is recognition of Iran's right to develop nuclear energy for peaceful purposes and the treatment of Iran in "the same manner" as other states under the Nuclear Non-Proliferation Treaty.
Iran would get help with developing nuclear power stations and be guaranteed fuel for them.
It would also be offered trade concessions, including the possible lifting of US sanctions preventing it from buying new civilian aircraft and parts.