Oil prices opened about 1% higher on Monday after the United States vowed to keep attacking Yemen's Houthis until the Iran-aligned group ends its assaults on shipping. The U.S. airstrikes, which the Houthi-run health ministry said killed at least 53 people, are the biggest U.S. military operation in the Middle East since President Donald Trump took office in January. Oil prices rose slightly last week, snapping a three-week losing streak caused by concerns over a global economic slowdown driven by escalating trade tensions between the U.S. and other nations.