The US has sanctioned a Shanghai-based manager of 35 product tankers as it continues its crackdown on Iran's oil industry.

Aoxing Ship Management Shanghai and its affiliate Sea Charming Shipping were blacklisted along with seven other companies and three individuals by the US Treasury's Office of Foreign Assets Control Wednesday afternoon.

Details on why the 12 were sanctioned are scant, with the US State Department — which called for the sanctions — only saying they knowingly engaged "in a significant transaction for the purchase, acquisition, sale, transport, or marketing of petrochemical products from Iran".