Indonesian authorities said that they seized an Iranian tanker and Panamanian tanker suspected of carrying out the illegal transfer of oil in their country’s waters.

The tankers — the Iranian-flagged MT Horse and the Panamanian-flagged MT Frea — were seized in waters off Indonesia’s West Kalimantan province.

The tankers are suspected of a variety of violations, including not displaying national flags, shutting off their identification systems, anchoring illegally as well as the illegal transfer fuel between ships and spilling oil.

Authorities were escorting the two tankers to Batam island in Riau Islands province for further investigation.

Iranian state television acknowledged the tanker’s seizure, citing Indonesian authorities. The report did not elaborate.

Iran, home to major oil and natural gas reserves, has seen its sales abroad deeply impacted by U.S. sanctions. This cut a crucial source of government revenue in Iran’s long-anemic economy.