The Obama administration is urging global port authorities to be on the watch for Iranian shipping vessels flying false flags or sailing under fraudulent registrations. Both practices violate international maritime law and sanctions.

The Treasury Department said Thursday that Iran's state-run shipping line, IRISIL, has been caught recently operating vessels whose registration from other nations has been revoked as a result of sanctions. As the number of such vessels rises, it said such incidents are likely to become more common. It urged port and canal officials should scrutinize the registry of Iranian vessels and take action if violations are found.

Treasury warned that maritime authorities or others in the shipping industry found to be assisting IRISIL evade sanctions would be subject themselves to U.S. penalties.