We have received a legal complaint from lawyers acting for Dmitry Firtash about this article. Mr Firtash complains that the contents of the article are untrue.

Gas supplies to Ukraine and EU states are linked to the Russian mafia, according to the US ambassador in Kiev.

His cable, released by WikiLeaks, followed statements by the then prime minister of Ukraine, Yulia Tymoshenko, to the BBC that she had "documented proof that some powerful criminal structures are behind the RosUkrEnergo (RUE) company".

Allegations have long swirled that the Russian crime don Semyon Mogilevich had covert interests in Swiss-registered RUE, which distributes gas from central Asia.

A billionaire Ukrainian businessman, Dmitry Firtash, nominally owns nearly half the company. (The Russian state firm Gazprom owns the other half.) In a confidential meeting with the ambassador, Firtash admitted Mogilevich was the real power behind his own multibillion-dollar gas interests. He insisted it was impossible to do business in Ukraine in the 1990s without striking sleazy deals with organised criminals.

In a secret memo the ambassador, William Taylor, wrote: "He [Firtash] acknowledged ties to Russian organised crime figure Semyon Mogilevich, stating he had needed Mogilevich's approval to get into business in the first place." Taylor said the "softly spoken" Firtash had come to see him on 8 December 2008 and "spoke at length about his business and politics in a visible effort to improve his image with the USG [United States government]".

His arguments were obviously "self-interested", Taylor said. "In a lengthy monologue Firtash described his evolution as a businessman from his beginnings as a food trader to the creation of RUE … He was adamant that he had not committed a single crime when building his business empire and argued that outsiders still failed to understand the period of lawlessness that reigned in Ukraine after the collapse of the Soviet Union."

Taylor grilled the oligarch about his alleged ties to Russian organised crime. The Ukrainian press had widely reported that RUE's circle of true beneficiaries included Mogilevich. Firtash countered that it had been impossible to approach a government official for any reason "without also meeting an organised crime member".

The cable stated: "Firtash acknowledged that he needed, and received, permission from Mogilevich when he established various businesses, but he denied any close relationship to him."

Other cables said the two men were closely linked through "joint holdings in offshore vehicles" and "mutual personal relationships". They also share the same lawyer.

[This update was inserted on 7 December 2010: Following publication of this cable, Dmitry Firtash issued a statement denying that Semyon Mogilevich had ever had any partnership or holding or other direct or indirect commercial associations or business interests with him. See footnote.]

During his frank conversation with the ambassador, Firtash launched an attack on Tymoshenko. At the time she was locked in a power struggle with her former Orange Revolution ally Viktor Yushchenko, who was president. Firtash called himself Yushchenko's "close friend" and confidant, and his anger was understandable. A month before he met the ambassador, Tymoshenko had reached a provisional agreement with the Russian prime minister, Vladimir Putin, to abolish all intermediaries in the murky gas trade with Russia. The deal – still unimplemented – was specifically drawn up to get rid of RosUkrEnergo.

The US says Mogilevich is one of the world's top mafia bosses, accusing him of creating eastern Europe's most powerful crime group in the 1990s. He is on the list of the FBI's 10 most wanted fugitives for his alleged part in a multimillion-dollar fraud involving a Pennsylvania-based company in the 90s.

The Russians arrested Mogilevich in 2008 over tax evasion at a cosmetics chain. But in 2009 he was mysteriously released. Russia's interior ministry described the charges against him as "not of a particularly grave nature". The cable is deeply embarrassing for Ukraine's new pro-Russian president, Viktor Yanukovych. Firtash, the founder and chairman of Group DF, is one of the main oligarchs who financed Yanukovych's rise to power this year.Firtash's business interests across Europe include energy, chemicals and real estate, and he owns an influential Ukrainian TV station.

In his cable Taylor noted that the gas trade had made Firtash staggeringly rich: "By 2006 Firtash's estimated worth was over $5bn, but most experts believed that Firtash had low-balled his true worth and estimated it was in the tens of billions. In his conversation with the ambassador Firtash gave no indication of the scope of his wealth."

• This article was amended on 7 December 2010 to update with a response issued by Dmitry Firtash. His full statement appears here http://www.groupdf.com/News_301.asp