Oil prices head for weekly rise on Middle East escalation

"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.