The United States is increasingly concerned about training by Iran's elite Revolutionary Guards for the Houthi rebels in Yemen, where Shiite militias continue to make territorial gains despite airstrikes by neighboring Saudi Arabia.

U.S. officials said Tehran's direct involvement with the Houthis was limited but that U.S. intelligence assessments had concluded that Islamic Revolutionary Guard Corps personnel were training and equipping Houthi units.

The officials, who spoke on condition of anonymity to discuss security matters, expressed concern that the IRGC's mission could include training the Houthis to use advanced weaponry they acquired after seizing Yemeni military bases.