WTI oil prices rose by 1.1 percent on Thursday to close at $51.7 per ounce posting a fourth straight day of gains, but analysts remained cautious about record-high U.S. crude inventories. The gains gave crude its highest close since a March 8 rout when investors bailed out of bullish positions due to concerns about supply. Crude prices have been rebounding in the last two weeks from that decline. Refinery runs are starting to increase as the U.S. summer driving season approaches and gasoline inventories have been declining. Yet U.S. government data still show crude inventories at record levels, prompting some analysts to grow concerned about speculators jumping back into the market after several weeks when they reduced positions in response to inventory figures.