A U.S. organization opposed to Iran’s nuclear ambitions has urged the Korea Register of Shipping to halt its services for key Iranian shipping companies.

United Against Nuclear Iran urged the South Korean firm in a recent letter to stop verification services for Iran’s state-run Islamic Republic of Iran Shipping Lines and its subsidiaries as well as the National Iranian Tanker Company.

Ships which have not been classified by companies such as the Korea Register of Shipping are not allowed to make port calls.

In response to the U.S. group’s call, the Korean firm said it is providing limited service to a small number of Iranian shipping companies in line with existing contracts.