A US safety firm has cancelled environmental and safety classification of two tankers following accusations that they had shipped cargoes of Iranian oil.

Reuters reported that the American Bureau of Shipping (ABS), the classification company that provides certifications services such as checking that ships are seaworthy, withdrew cover for the two tankers in the past month.

Without the certification -- the so-called class cover that includes vessel safety inspections -- vessels are unable to secure insurance cover or call at most international ports.

Advocacy group United Against Nuclear Iran (UANI) that tracks Iran-related tanker traffic through ship and satellite data, informed the American Bureau of Shipping about the breach of US sanctions in December.

According to correspondence between the UANI and ABS seen by Reuters, several vessels were involved in the transfers of Iranian oil, including the Panama-flagged Karo owned by Hong Kong-based Delta Lines and the Belize-flagged Elsa owned by India-based Karo Shipping Services.

The ABS, the only US firm among the top 12 ship classification societies that are mainly based in Europe and Asia, withdrew the cover for Elsa on December 17 and the Karo on January 13 because of their illicit transshipment operations with Iranian tankers.

"Classification societies are faced with the challenge of keeping up with Iran's tactics in order to avoid facing sanctions themselves," said UANI Claire Jungm