Iran's National Petrochemical Company says it is being excluded from buying a major Dutch plastics maker by US political pressure.
Iranian goods are banned from the US by trade sanctions
Polypropylene maker Basell is being sold by its joint owners, Anglo-Dutch oil group Shell and chemicals giant BASF of Germany.
INPC has been "unofficially told Iran cannot buy Basell", the ISNA news agency cited the firm's boss as saying.
A Shell spokeswoman in London said the Basell negotiations were confidential.
She declined to confirm whether the Iranian firm was among the bidders being considered, or comment on press reports that US officials had raised objections.
US firms face trade sanctions if they import Iranian goods.
Basell is the world's biggest maker of polypropylene and its US customers are worried about the implications of an Iranian deal, according to Reuters news agency and the Wall Street Journal.
US sanctions date back to tensions following Iran's 1979 Islamic revolution. But diplomatic tensions with the US and Europe are currently escalating over Iran's nuclear programme, making a change of policy unlikely.
"Shell and BASF announced that we had an intention to review strategic alternatives for Basell in July 2004. Offers for the company have been received and advanced discussions have taken place," she said.
"We are bound by commercial confidentiality and I cannot give you any further comment."
A deal to sell Basell for 4.4bn euro (£3bn; $5.7bn) is reportedly close to completion. Its products are used in everything from plastic bottles to car parts.
"Although NPC won all aspects of the Basell tender, due to US pressures, we cannot buy Basell," ISNA quoted the Iranian firm's managing director Mohammed Reza Nematzadeh as saying.
An Indian firm, Haldia, is now poised to win the bidding for Basell, the Wall Street Journal reported. Haldia is backed by two New York-based financiers, Chatterjee Group and Access Industries.
The US is "concerned that Basell is a very large multinational with a high degree of technology and Iran is state sponsor of terrorism and a proliferator," Reuters cited a US State Department official as saying.
"The chances of it being the Iranians is decreasing," Reuters quoted a separate, unnamed source close to the negotiations as saying earlier in the week.
"There would be a risk to Shell's reputation if they were to scrap a potential deal with Iran, because it is developing energy sector interests in that country."
Iran was one of the countries listed by President George W Bush as belonging to an 'axis of evil' in a speech in 2002.