At least 24 cargo ships carrying a total 480,000 tonnes of wheat and other grains are sitting off Iran's coast, unable to unload because of the effects of sanctions on the Islamic republic, an industry source said.

EU and US sanctions barring contact with Iran's financial sector have caused letters of credit to dry up, forcing grain suppliers to stay away from unloading docks until they can be paid, the source said on condition of anonymity.

"Transfer of money through banks is almost impossible," said the source in the import sector.

Iran imports around 4.5 million tonnes of grain a year, according to the International Grains Council, most of it maize from Brazil, Argentina and Ukraine, mainly for animal feed.

Its farmers produce around 21 million tonnes of grain per year, according to the UN Food and Agriculture Organisation.

The stalled shipments showed the impact Western sanctions imposed over the past three months.

The United States and Europe have applied the measures in an effort to pressure Iran to give up its nuclear activities, which they fear include research for atomic weapons, despite Tehran's denials.