By Warren Strobel and Mark Hosenball WASHINGTON, March 27 (Reuters) - The United States is increasingly concerned about training by Iran's Elite Revolutionary guards for the Houthi rebels in Yemen, where the Shi'ite 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 a**nymity to discuss security matters, expressed concern that the IRGC's mission could include training the Houthis to use advanced ******ry they acquired after seizing Yemeni military bases. Saudi Arabia, which launched aerial bombardments of Houthi forces this week, has said the militia was receiving extensive backing from Iran, the kingdom's regional rival.