Advocacy group United Against Nuclear Iran (UANI) has released a satellite photo of two oil tankers during ship-to-ship transfer of Iranian crude reiterating calls for sanctioning them.

UANI Chief of Staff Claire Jungman said on Saturday that after clouds cleared in the South China Sea they managed to spot the vessels that were transferring Iran’s crude oil east of Singapore.

“TIFANI and VORAS, two vessels we've repeatedly called for sanctions on, are feeding vessels that are spoofing their automatic identification system (AIS) transponders”, she said in a tweet with the image.

TIFANI is currently sailing under the flag of Tanzania and VORAS is Guyana-flagged and owned by Malaysia International Shipping Corporation Berhad.

The news came just a day after Reuters reported that a US safety firm has cancelled environmental and safety classification of two tankers after UANI alerted the classification company that they had carried cargoes of Iranian oil.

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.

UANI that tracks Iran-related tanker traffic through ship and satellite data, informed the American Bureau of Shipping, the company that provides such certifications services, about the breach of US sanctions in December.