Iran's Oil Minister Bijan Zanganeh launched a blistering attack Monday on industry middlemen, branding them "parasites that want to suck the nation's blood" for their own financial benefit.

His comments, a day after the United States and Europe moved to lift sanctions on Iran following a July nuclear deal, coincide with ramped up efforts to attract international oil majors back to Tehran.

Sanctions relief could unlock Iran, which has the world's largest combined oil and gas reserves, from decades of under-investment and stifled production.

But Zanganeh urged foreign investors to deal directly with the oil ministry, rather than using third parties to find their way into the Islamic republic's abundant energy market.

"We despise the corrupt parasites that want to suck the nation's blood even in this situation," he said, to loud applause at an oil and energy industry event in the capital.

Iran will hold a conference in Tehran on November 21-22 to unveil new contract terms designed to tempt foreign companies back.

Despite renewed interest since the July 14 nuclear deal between Iran and Britain, China, France, Russia and the United States plus Germany, the precipitous fall in international oil prices in the past year may cause energy firms to hold off.

A raft of international sanctions on Iran -- as punishment for its disputed nuclear programme -- led energy giants such as Total, Shell and Statoil to leave in recent years.

Zanganeh said the "sanctions era" was almost over, indicating the practice of middlemen profiting from deals because direct business with the ministry had been avoided would also come to an end.

He also tried to reassure potential investors over direct dealings with the government.

The government is "in a relentless fight against corruption" and those foreigners waiting to leap into Iran's oil sector should know that "utmost transparency" is a priority, Zanganeh said.

"I recommend foreign companies stay away from these corrupt individuals," who know nothing but "deceitfulness", he said of middlemen.

They "became much better at fraud" during sanctions, Zanganeh warned.

"They will tell you that until you give us our commission you can't get your work done. Don't believe them."

Instead he promised a new regime that would include better access to himself and senior oil managers in Iran.

"Think long-term and work directly with us. Corruption will be revealed eventually and will ruin both your business and credibility."

Iran's ageing and under-tapped oil industry is desperate for foreign technology and funding.

A mere one-percent increase in efficiency would bring in $400 billion in added value, according to Gholam Reza Manouchehri, the chairman of Monday's event.

After next month's Tehran conference, Iran will hold an international event on February 22-24 in London, by when all nuclear-related sanctions should have been lifted.