US President George W Bush is to hold talks with European Union leaders later on Tuesday as they meet in Slovenia for their twice-yearly summit.
The BBC's Oana Lungescu in Ljubljana says they are expected to issue a joint warning to Tehran that more sanctions against Iran's banks are being readied.
They are pressuring Iran to abandon its nuclear programme, rejecting claims it is solely for peaceful purposes.Carrot and stick' policy
There will be a tough joint message to Tehran about further sanctions against Iranian banks linked to nuclear proliferation and terrorism, our correspondent says.
Mr Bush spoke of the work to be done in Afghanistan, ahead of his trip to Europe
Mr Bush is also expected to apply pressure on individual European businesses to take a harder line.
Barclays Bank, based in Britain, has already responded to US pressure, and ended all dealings with Iran's Saderat Bank and Bank Melli.