Oil prices have been elevated since Monday afternoon, after it became clear the United States and Iran had not negotiated a lasting peace deal. The memorandum of understanding signed by the two countries had called for a deal to be in place by now.

By Robert Harvey LONDON, Aug 18 (Reuters) - Oil prices traded slightly higher for a third session on Tuesday as prospects receded for a deal to end the Middle East war, with Iran saying it would adopt a more offensive stance and the U.