DUBAI — A U.S. campaign to throttle Iran's economy by blockading 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. Although Iran's clerical rulers managed to bypass sanctions for decades, the latest U.S. moves have left them in a far more sensitive position, with few remaining channels to secure foreign currency or buy goods, the sources said. In particular, the effort to stop Iran accessing international financing networks in other countries it has long used to keep its economy functioning poses a real and urgent threat, the sources said. Any sign the economic campaign may break a months-long deadlock in the conflict will likely cheer U.S. planners, though Iran has also warned it could respond to the pressure with military escalation, raising the stakes at a key moment. Petrol stocks low as currency collap