LONDON - A Russian firm has decided to stop verifying safety and environmental standards for one of Iran's biggest shipping groups, a letter showed, the latest international company wary of being caught up in Western sanctions on the Islamic Republic.

Iran is under growing pressure over its disputed nuclear program and companies are cutting ties with its vital shipping sector, which transports most of its crude oil, for fear of losing lucrative US business.

Moscow opposes concerted trade pressure on Tehran by Washington and its allies, making the Russian company's decision unusual.

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