Four major energy companies are pulling out of Iran in order to comply with US sanctions, the state department says. Royal Dutch Shell, Total, Statoil and Italy ENI will now avoid US penalties targeting companies which do business with Iran. Naftiran Intertrade Co, based in Switzerland, will face new sanctions . The US has tightened sanctions on Iran over concerns about its nuclear ambitions, which Washington fears are aimed at acquiring atomic weapons. Tehran insists its nuclear programme is for purely peaceful purposes. Recent legislation gives the US administration the power to penalise foreign companies which invest more than $20m in Iran's energy sector. However, companies taking steps to comply with the law are exempted from penalties.