The last big company doing classification work on Iranian ships, key to securing insurance and ports access, said on Friday it is stopping the work, spelling further difficulty for Iran's shipping, including its oil exports.

The Korean Register of Shipping (KR) is the last of the world's top 13 classification societies to halt marine work in Iran following a recent exodus, including Britain's Lloyd's Register, triggered by Western sanctions on Tehran.

IACS members such as Germany's Germanischer Lloyd and France's Bureau Veritas pulled out of Iran after being urged to exit by United Against Nuclear Iran, which includes former U.S. ambassadors on its board and is funded by private donations.

The pressure group backs tougher sanctions on Iran.

"We applaud KR for this responsible and significant decision," a spokesman for the group said on Friday.

"KR has joined other shipping services and countries in forcing Iran's business partners to now assume most all of the shipping risks for their business with the regime."