The U.S. is anxious that any production cuts agreed by OPEC, on top of its own sanctions on Iran, will limit supply and prompt global oil prices to rise and hit its own consumers hard. President Donald Trump has tweeted numerous criticisms of OPEC, most recently on Wednesday when he said "hopefully OPEC will be keeping oil flows as is, not restricted."
Showing the extent of its concern, the U.S. sent its special representative for Iran, Brian Hook, to Vienna to speak to Saudi Energy Minister Khalid al-Falih met on Wednesday, ostensibly to discuss OPEC's direction on oil output cuts.
Iran is not happy about Hook's arrival in Vienna and Zanganeh joked about the U.S. trying to join OPEC.
On Thursday, Zanganeh said: "I thought he was coming here to join OPEC. But if the U.S. doesn't want to join OPEC, it's unprofessional behavior to come here and have meetings on some member countries and to put pressure on them about what to do. It comes after the tweet from President Trump instructing OPEC what to do and not to cut production."
"I'm sure most OPEC members cannot accept taking instruction from the president of the U.S. on what to do," he added.
Hook's arrival in Vienna is telling for RBC Capital Markets' Global Head of Commodity Strategy Helima Croft who said geopolitics are "clearly intruding into the OPEC proceedings."
"The fact that the U.S. special envoy for Iran is reportedly in Vienna at the moment means that the issue of sanctions will cast a significant shadow over the discussions," she told CNBC Wednesday, adding that Hook's decision to meet Al-Falih "before he meets with anyone else from OPEC is causing considerable consternation with certain delegations."