WASHINGTON, Feb 5 (Reuters) - The head of the U.S. Federal Aviation Administration told Congress Monday the agency opposes raising the mandatory airline pilot retirement age to 67 from 65 saying the agency should be allowed to first conduct additional research.

"It is crucial to provide the agency an opportunity to conduct research and determine mitigations," FAA Administrator Mike Whitaker said in a letter seen by Reuters. The U.S. House in July voted 351-69 on a aviation reform measure that would hike the mandatory retirement age to 67. (Reporting by David Shepardson)