Exports to Iran rose to $199.5m in the first eight months of 2012, from $150.8m during the same period last year.

US exports to Iran have jumped by nearly a third this year, despite the tightening of financial sanctions.

According to the US Census Bureau, exports to Iran rose by 32 per cent to $199.5m in the first eight months of 2012, from $150.8m during the same period last year.

The largest category of US exports through August was $89.2m in sales of wheat and other grains. In 2011, the US sold $21m worth of corn to Iran.

Sales of dairy products and other related exports more than doubled to reach $20.3m from $7.8m.

Medical, dental, surgical and other “electro-diagnostic apparatus” sales also rose to $8m from $4.7m.

The data is surprising given Western efforts to isolate Iran economically, because of its suspected pursuit of nuclear arms.

The increase masks a drop in the export of some humanitarian goods such as medicines, a decline US exporters blame largely on the difficulty of getting paid by Iranian importers because of the US financial sanctions.

Medicinal and pharmaceutical products, including those sold in bulk and those for animals, fell to $14.9m from $26.7m.

Although Iran can still import such goods, US companies have complained for months that it is harder and harder to get paid because Iran’s big banks have been blacklisted by the US Treasury, for alleged support of terrorism or involvement in its alleged weapons of mass destruction programmes.

Blacklist

While some Iranian banks are not blacklisted, these tend to be smaller institutions with limited access to foreign exchange.

The possibility that one of these might obtain hard currency from a blacklisted Iranian institution has scared foreign banks, who fear that they might be accused of having “indirectly” dealt with a designated Iranian bank, sanctions lawyers say.

As a result, groups ranging from religious-affiliated non-profits to liberal members of Congress to the US Dairy Export Council have argued for ensuring that the banking sanctions do not choke off humanitarian trade.

“The Administration’s sanctions against Iran have created a de-facto humanitarian banking blockade,” said Kate Gould, legislative associate for Middle East policy at the Friends Committee on National Legislation.

Shawna Morris, vice president for trade policy at the National Milk Producers Federation & US Dairy Export Council, said her group had long supported a humanitarian exemption.