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HOUSTON, July 16 (Reuters) - Grades were broadly unchanged on Thursday, dealers said, even amid fears that Yemen-backed Houthis could close the Red Sea oil route if the United States strikes Iranian power infrastructure, further tightening supply.

Iran and the United States exchanged intensifying fire on Thursday in a week-long escalation that has largely unraveled last month's truce, while Tehran disputed President Donald Trump's claim that a U.S. citizen had been released.

Iran has asked Yemen’s Houthi movement to stand ready to close the Red Sea oil route if the United States strikes Iranian power infrastructure, three sources told Reuters on Thursday, posing a potent new threat to global energy supplies.

Fewer vessels travelled through the Strait of Hormuz on Wednesday, the first day after the U.S. reimposed its naval blockade on Iranian ports with both countries escalating strikes across the Gulf, shipping data showed.

Iraq briefly suspended oil loadings on Thursday after a drone hit an oil tanker at its Basra terminal, four Iraqi oil and security sources told Reuters, before later resuming them.

In the United States, the Energy Information Administration said it was reviewing the results of a pilot study to better understand minimum working crude oil inventory for some storage hubs at Cushing, Oklahoma, after stocks reached levels that may constrain normal operations.

In refining news, Citgo reported an unexpected shutdown of a third-party fuel gas processing facility at its 165,000 barrels per day Corpus Christi, Texas refinery's West plant, according to a Texas Commission on Environmental Quality filing.

• Light Louisiana Sweet for August delivery eased to a midpoint of a $1.50 premium and was seen bid and offered between a $1.00 and $2.00 a barrel premium to U.S. crude futures CLc1

• Mars Sour firmed to a midpoint of a $1 premium and was seen bid and offered between a 75-cent and $1.25 a barrel premium to U.S. crude futures CLc1 ​

• WTI Midland eased to a midpoint of a 20-cent premium and was seen bid and offered between parity and a 40 cent​ a barrel premium to U.S. crude futures CLc1 ​

• West Texas Sour eased to a midpoint of a $3 discount and was seen bid and offered between a $3.25 and $2.75 a barrel discount to U.S. crude futures CLc1 ​

• WTI at East Houston, also known as MEH, traded between a 25-cent and 65-cent a barrel premium to U.S. crude futures CLc1

• ICE Brent September futures LCOc1 remained unchanged at $84.23 a barrel on Thursday.

• WTI August crude CLc1 futures rose 71 cents and are currently trading at $79.66 a barrel on Thursday.

• The Brent/WTI spread widened 12 cents to last trade at -$5.95 after hitting a high of minus $5.46 and a low of minus $6.25