BEIJING/SINGAPORE, July 24 (Reuters) - Oil futures prices retreated on Friday but are still set for hefty weekly gains because of concerns about disrupted energy flows in the Red Sea and fears of further escalation in the U.S.-Israeli war on Iran.

Brent futures fell $1.12, or 1.11%, to $99.55 a barrel at 0648 GMT, having settled up 7% above $100 in the previous session for the first time since May, after Iran-aligned Houthis said they struck two Saudi oil tankers in the Red Sea.

The contract remained on course for a 13% advance this week.

West Texas Intermediate (WTI) futures was down $1.12 or 1.21% at $91.06 a barrel, on track for an 10.4% weekly rise.

"The potential supply disruptions facing the market now are larger than at any time during the war," ING analysts said in a note on Friday. "Not only have oil flows through the Strait of Hormuz essentially dried up, but there are clear risks to Saudi oil flows from the Red Sea."

"Further escalation in the Gulf and fears of a widening conflict are putting a significant amount of oil supply at risk."

The number of tankers crossing through the Strait of Hormuz fell to just one on Thursday, the lowest since May 7, according to ship-tracking data from Kpler, an analytics firm.

The Bab el-Mandeb shipping route controls access from the Red Sea to the Indian Ocean and is the second-most-important oil channel after the Strait of Hormuz.

U.S. President Donald Trump vowed to "hold Iran responsible" for any further attacks.

The Iran-aligned Houthis had declared on Monday that they were imposing a naval blockade on Saudi Arabia, which had been diverting its oil via pipeline to get around Iran's closure of the Strait of Hormuz.

Iran had been pressing the Houthis to close the Bab el-Mandeb gateway to the Red Sea if the U.S. continued to attack Iranian power infrastructure, after an interim truce between the two countries collapsed two weeks ago.

Also on Thursday, Kazakhstan's energy ministry said oil companies temporarily cut back production after suspected Ukrainian drone attacks forced the country's main Black Sea export terminal to close.

The Caspian Pipeline Consortium stopped receiving oil from Kazakhstan after suspending loadings because of attacks on tankers at the terminal, industry sources said on Tuesday. The route handles about 2% of the world's daily crude supply.

Kazakhstan's energy ministry did not specify the scale of the production reductions, but one source said the country's biggest field had cut output by more than half.