Tougher measures imposed by the European Union and the United States have tightened the screws on Tehran, which relies on its shipping trade for many imports including food, consumer and industrial goods. Many foreign companies, including shipping firms, have pulled out for fear of losing business in the US and due to the complexities of arranging non-sanctioned deals.
Despite the setbacks, industry sources say producers in Ukraine are providing Iran with coking coal, also known as metallurgical coal, and coke - key steel ingredients.
"Iranians used to buy a lot of coking coal from Australia to make their own coke but that has stopped now as the big companies there don't want to do it as they are too exposed," a British-based coal trade source said. "So Iran went to buy coke from Ukraine," he added, referring to the concentrated coal used in blast furnaces.
While coal is not directly targeted as a commodity, the European Union imposed a ban on steel sales to Iran last week, making the Islamic Republic's coal needs more pressing because it now must produce more steel itself.
"Iran is one of the fastest-growing countries in terms of steel production so they need more steel raw materials," a European based trade source said. "They need to import more (metallurgical) coal and coke," he said.
Lured by a trade worth nearly $25 million a month, suppliers in Ukraine are aiming to take advantage.
"The US and EU sanctions programs currently in place against Iran are complex and include sanctions against the indispensable marine insurances," said Jakob Larsen with BIMCO, the world's largest private ship owners' association.
"As is often the case, for those who are willing and able to take the risk, the rewards are more likely to be high. In such a market the risk-taker segment will try to find a way out."
Sources say the trade is complex involving often multiple brokers and diverse payment arrangements including a mix of currencies such as Russian rubles.
"We have been approached to sell some (metallurgical) coal to Iran and they have been buying more lately," one Ukrainian metallurgical coal producer said. "We have done some business but not directly, through another country -- Syria and Lebanon," he said, without providing further details.
Even those looking to do deals with Iran from Ukraine are having to find creative ways to trade, other sources said.
"One of the ways around it being looked at is barter. We've been approached several times but haven't done any deals yet to do barter of coal for steel of equivalent value, that way no money needs to change hands," a raw materials trader said.
A Black Sea based trade source also reported receiving multiple enquiries in recent weeks from Iran. "It isn't easy, it's very complicated to deal with Iran," the source said.
"To do some business there you must use a bank with specialist knowledge, not the usual banks or Russian banks. I would use a Lebanese bank instead, which has representative offices in Tehran and acts as an agent between the mills and suppliers."
Trade sources said it was unclear who the ultimate Iranian end-users of imported material were because of the involvement of agents and middle men and the desire to conceal purchases.
"Anybody who is doing this kind of business is not going to say who the buyers are," another Ukrainian coal source said.
Another Black Sea based industry source familiar with the shipments said cargoes were being routed from the cargo port of Nikolayev, not far from Ukraine's larger terminal of Odessa.
"Exports have been going on a constant basis already for two years, and there are around two to three cargoes a month," the source said. "Iranian vessels come into the port, pick up the coal and then head for home."
A Nikolayev port spokeswoman said: "There were no coal shipments to these directions (the United Arab Emirates and Iran) during the past four weeks," without giving further details.
Official data from Ukraine's Statistics Service showed overall exports to Iran in the period from January to August of this year rose 10% to $800 million compared with the same period last year.
Coal exports in the period reached $421 million, just over 20% lower than in the same period last year.
Trade sources say the figures do not reflect the full extent of the trade and between 170,000 to 200,000 metric tons a month of coal are exported from Ukraine, especially using vessels belonging to Iran's top cargo firm, the Islamic Republic of Iran Shipping Lines (IRISL).
"IRISL vessels work out cheaper than using foreign ship owners, who are charging massive premiums for to Iran," the second Black Sea trader said. "IRISL increasingly is finding it has fewer trade options, so it works out for Iran well."
An IRISL official in Tehran said: "Maybe sometimes we have some offers on coal for factories in Iran."
IRISL has been on a Western blacklist of sanctioned entities for years, accused of transporting weapons, which it denies. It has tried to dodge sanctions by using various tactics including changing its flags, and setting up front companies, the US Treasury and the European Union have said.
Other tactics have included changing ownership of ships and flags and falsifying cargo documents in a bid to become more invisible, the US Treasury has said.
IRISL's chief said this week that if sanctions pressure continued the carrier, which is receiving state help, would face grave problems. It has already had $50 million dollars blocked by the central bank – reflecting the acute shortage of US currency and underscoring the problems with trades involving Iran's fleet.
Iranian President Mahmoud Ahmadinejad has faced growing criticism over his handling of the economy, especially after the Iranian currency plunged by more than a third in recent weeks.
"The regime has reason to be concerned about the budget and being able to support subsidies in the long term as its earnings from trade continue to diminish," said Anthony Skinner of risk analysts Maplecroft.
"President Ahmadinejad has received a lot of blame for the state of the economy and economic mismanagement partly explains the dire state of affairs."