* Western banks cold shoulder trade finance deals
* Several vessels already been held up for weeks -sources
* Iranian official says banks "afraid of doing business"
LONDON/ANKARA, May 11 (Reuters) - Payment problems are disrupting commercial food cargoes to Iran, with hundreds of thousands of tonnes of grain and sugar stuck in transit, as Western banking sanctions complicate deals and trade financiers scale back exposure.
Iran is not barred from buying food or other "humanitarian" goods under sanctions imposed over Tehran's pursuit of nuclear technology, but measures by the European Union and the United States have made trade more difficult over the past two years.
Several international trade sources, with knowledge of deals that have been affected, told Reuters that ships carrying cargoes of grain, including wheat and soybeans, as well as raw sugar, have been stuck for several weeks outside Iranian cargo ports such as Bandar Imam Khomeini and Bandar Abbas.
With evidence of people starting to stockpile food and prices rising following cuts in government subsidies, Iranian officials acknowledged to Reuters that there are import problems, notably due to reluctance among international banks.
One European trade source said: "There are problems getting paid on deals and Iran looks to be struggling on the trade finance side. It comes down to the banking complexities, which have held up cargoes for a number of suppliers."
Several trade sources point to growing difficulties opening letters of credit, vital to ensuring smooth delivery of goods.
"Western banks are unwilling to get involved," a second European trade source said. "As soon as the banks see the word 'Iran' in the paperwork, you get it rejected."
Bankers and government officials said Western lenders are steering clear of attempts by Iran to get them involved in financing humanitarian transactions, fearing they could be penalised under U.S. sanctions.
"We still cannot open letters of credit," said one Iranian government official, who spoke on condition of anonymity.
"International banks are concerned about - or let's say afraid of - doing business with Iran," the official added. "And we don't know what the solution will be. Dozens of ships are waiting at the ports and we just can't do anything."
Reuters ship tracking data shows that five panamax-sized vessels, each capable of carrying 60,000 to 70,000 tonnes of grain, reached Iran in early April from ports in Europe, Australia and South America. The vessels were still located around Bandar Imam Khomeini and Bandar Abbas on May 9, several weeks after they should have been able to discharge a cargo.
Reuters was unable to reach owners of the five vessels, which all sailed from international wheat terminals, including Australia's Kwinana and Rostock and Hamburg in Germany.
"Having a panamax at port for that long would suggest to me there has been a payment issue," said a Sydney commodities trader. "There have been some issues with Iran in the past."
The five cargoes would represent nearly 2 percent of Iran's estimated annual wheat consumption of around 17 million tonnes.
Ship tracking data indicated that other vessels may also have been disrupted in recent months. Four panamax-sized vessels left Australia for Iran at the end of January.
After arriving in Iranian waters in mid- to late February, the vessels were stuck mainly around Bandar Imam Khomeini until they departed in April. A spokesman at agribusiness group Cargill's Australia office said three panamaxes it had been involved with had been delayed due to port congestion in Iran.
A separate panamax vessel originating in Ukraine was also stuck in Iran for weeks until April. A further three smaller vessels, including one carrying raw sugar, were also held up for weeks, ship tracking data showed.
A European trade source said at least one or two cargoes had been re-routed to other buyers: "The shipments in question were stuck in Iran for a long time and clocked up a big loss," the source said. "They were subsequently traded to other buyers."
A second Iranian government official said ships were waiting with perishable goods: "A few have had to return to the country that we purchased goods from.
"Various factors are involved - like Iran not facilitating the delivery and also the banking problems."
A spokeswoman for U.S. agribusiness company Archer Daniels Midland, which has supplied Iran, said many international banks would not participate in transactions with Iran "for fear of being sanctioned or fined".
"Another hindrance is Iran's foreign currency controls," she added. "Ships arriving in Iran with grain must frequently wait weeks for the Central Bank of Iran to approve the release of funds to pay for the cargo."
Banking and trade finance sources said several Iranian banks were cut off from the global electronic cross-border payment system SWIFT, which was adding to further financing problems.
A banking source said: "We should not rule out further bureaucratic delays in Iran to manage their limited availability of hard currency until sanctions are properly eased."
(Additional reporting by Michael Hogan in Hamburg, Colin Packham in Sydney and Nigel Hunt in London; Editing by Veronica Brown and Alastair Macdonald)