Three Hong Kong-based trading companies and one United Arab Emirates firm were hit with US Treasury sanctions for helping Iran's Persian Gulf Petrochemical Industry Commercial Co. ship "millions of dollars worth" of petroleum and petrochemical products to unnamed East Asian buyers, the Treasury said in a statement.

The companies were part of "an international sanctions evasion network" helping Iran gain export earnings as it struggles under international economic pressure, according to the Treasury.

In addition, the US State Department blacklisted two shipping firms, one based in Qingdao, China and the other in Singapore, for helping arrange the shipments of the petrochemicals to buyer.

A tanker ship controlled by the Qingdao firm, the Panama-flagged Glory Harvest, was also placed on the blacklist.

The sanctions block any assets the entities own under US jurisdiction and ban US persons or entities from doing business with them, effectively constricting their access to the global financial system.