The US Thursday imposed sanctions on prominent Turkish businessman Sitki Ayan and his network of companies for violating oil export sanctions on Iran.

The department accused Ayan of acting as a facilitator for oil sales and money laundering on behalf of Iran's Revolutionary Guard Corps.

Ayan's companies have established international sales contracts for Iranian oil, arranged shipments and helped launder the proceeds and obscured the origin of the Iranian oil on behalf of Iran's Quds Force, an arm of the IRGC, the Treasury said in a statement first reported by Reuters.

"Ayan has established business contracts to sell Iranian oil worth hundreds of millions of dollars to buyers," in China, the United Arab Emirates and Europe, the statement says, adding that he then funneled the proceeds back to the Quds Force.

Ayan's son Bahaddin Ayan, his associate Kasim Oztas and two other Turkish citizens involved in his business network are also designated, along with 26 companies including his ASB Group of Companies, a Gibraltar-based holding company and a vessel.

The Treasury action freezes any US assets of those designated and generally bars Americans from dealing with them. Those that engage in certain transactions with those designated also risk sanctions.

The US measures come at a time when ties between the United States and Turkey are strained over a host of issues, including disagreement over Syria policy and Ankara's purchase of Russian air defense systems.

Washington maintains sweeping sanctions on Iran and has looked for ways to increase pressure as efforts to resurrect a 2015 nuclear deal with Tehran have stalled.