Bushehr nuclear reactor - 29 Sept 2010Iran insists its nuclear ambitions are purely peaceful, but the US fears Tehran is pursuing weapons

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.