U.S. and European sanctions are taking their toll of the Iranian economy, even though Tehran has become adept at getting around them to sell its oil.

But most of its sales channels have been closed and these days it has to pay through the nose for vital imports, such as wheat, as the effects of the sanctions first imposed two years ago throttle key sectors of the economy.

"The extra costs, which traders in Geneva, Dubai and Tehran estimate at between 5 and 10 percent above prevailing prices in the international market, are putting further pressure on the country's finances," the Financial Times reported.

The sanctions have been increasingly expanded and tightened over the last 18 months, with the United States and the 27-member European Union recently imposing new measures targeting Iran's oil and banking sectors.

On Thursday, in a separate package, the U.S. Treasury stepped up pressure on Tehran to abandon its contentious nuclear program, which is the core issue in this economic war, by imposing sanctions on seven companies and five individuals involved in that program.

Banks, companies and individuals who have dealings with those targeted would lose all access to the U.S. banking system.

These sanctions followed a November report by the International Atomic Energy Agency that Iran continued to defy the United Nations and the world community by refusing to slow its uranium-enrichment program which the West contends is aimed at producing nuclear weapons. Iran denies that.

Most of Iran's channels for exporting its oil, its economic jugular, have been pinched off by the sanctions first imposed in mid-2010. But some Asian customers are importing clandestinely.

On Dec. 15, Iranian Economics Minister Shamseddin Hosseini underlined how deeply sanctions were biting when he told Iranian state television that Tehran's oil revenues had been halved from 2011.

Iranian lawmaker Mohammad Reza Bahonar disclosed in September that oil exports had nosedived as low as 800,000 barrels per day in July from 2.3 million-2.4 million bpd in 2011.

But in recent weeks, shipping and oil industry sources say, the Iranians and their middlemen have pushed up export levels by disguising their oil, transferring cargoes at sea to other vessels or unloading at remote ports from where the Iranian oil is transported to Asian markets aboard foreign-flagged vessels.

"For an economy that's still 90 percent dependent on hydrocarbons for generating foreign currency earnings, guile and subterfuge have become unofficial instruments of policy," the Middle East Economic Digest reported.

One dodge the Iranians use is to disguise their crude with oil from neighboring Iraq, which has similar specifications to Iranian crude.

The Iraqi oil is smuggled by truck across the border where it becomes what traders have dubbed "Iraqi special blend" that's sold in Persian Gulf ports for shipment to Asia.

These transactions are reportedly handled largely by small companies based in the United Arab Emirates, 200 miles across the gulf from Iran and a major smuggling route between them.

Most of these under-the-counter deals are conducted in the shipping and oil storage hub of Fujairah in the southern Emirates and Hamriyah, the sources reported.

These tactics were largely responsible for a recent uptick in exports of Iranian fuel oil, used for electricity generation and to drive ships, despite a July EU ban on oil and shipping insurance that badly hit regular exports.

Between July and October, Iran sold an average of 648,000 tonnes of fuel oil, up from 636,000 tonnes for January to June.

That earned around $410 million a month, allowing Tehran to recoup some of the $3.8 billion it's shed in crude exports since July.

The sanctions have also blocked technology that Iran's oil industry needs to arrest the decline in production levels.

This could have more far-reaching consequences for Iran's rundown oil industry, long starved of new technology to extend the lives of mature fields.

MEED reports the National Iranian Oil Co. "wanted to tap foreign investment to support its fifth development plan, from 2009-14, for development and exploration activities, with a projected total spend of $150 billion. Little of that has been forthcoming ...

"Without a change in the political situation, Iran is facing the long-term deterioration of its oil assets and the stunting of the enormous potential of its still largely untapped natural gas reserved."