A South Korean ship classification society faces pressure from a U.S. lobby group to stop verifying safety and environmental standards for Iran's biggest shipping companies as tightening U.S. and European sanctions restrict its oil exports, Reuters said.

Without verification from such bodies, ships are unable to call at international ports.

U.S. group United Against Nuclear Iran (UANI), in a letter sent this week to the Korean Register of Shipping (KR), urged the society to stop providing classification and certification services to the Islamic Republic of Iran Shipping Lines (IRISL), its front companies and the National Iranian Tanker Company (NITC).

A targeted campaign by UANI, which includes former U.S. ambassadors on its board and is funded by private donations, had led Germany's Germanischer Lloyd and France's Bureau Veritas to drop their classification cover for Iranian shipping firms.

A KR official said on Thursday, June 28 the society was looking into all measures.

"KR provides limited classification services to a small number of Iranian ships as part of its ongoing contractual responsibilities," it added in a statement.

"KR continues to monitor the situation concerning Iran and closely follows all relevant international regulations. KR's policy is to comply, without compromise, with all national and international regulations concerning Iran."

Classification societies are hired by ship owners to regularly check that vessels, from their hull and propulsion systems to the machinery and appliances, meet international safety standards. Under international conventions, a classification is required for a ship to dock at major ports.