US pressure on Iran starting to tell, as sanctions and blockade bite

(Widens distribution) By Parisa Hafezi and Angus McDowall DUBAI, Sept 3 (Reuters) - A U.S. campaign to throttle Iran's economy byblockading its oil exports and stopping sanctions evasion is growing increasingly difficult to withstand, three senior Iranian sources said. Washington has in recent weeks sought to ratchet up the economic pressure on Tehran, in an effort to extract concessions in any future negotiation that six months of conflict have so far failed to secure.