"There was a fire [and it is] unclear at this point whether or not the fire is what caused the hard landing or whether to not the fire broke out after," said Army Col. Steve Warren. "There were injuries associated with the fire."
The Osprey suffered a "mishap" during the 11:40 a.m. landing aboard Marine Corps Training Area Bellows at Marine Corps Base Hawaii, said Capt. Brian Block, a spokesman for the 15th Marine Expeditionary Unit.
Twenty-two Marines were aboard the tiltrotor aircraft that takes off and lands like a helicopter and flies like a plane. The 21 injured Marines were taken to local hospitals for assessment and treatment, Block said.
Early reports from the scene indicated only 12 Marines suffered injuries.
Block described the exercise as routine sustainment training following the MEU's departure from San Diego on May 10 for a seven month deployment to the Middle East and Asia-Pacific region. The incident occurred on the island of Oahu, home to Honolulu and Pearl Harbor.
Residents in the vicinity reported seeing heavy, black smoke rising from the site.
The cause of the incident remains under investigation, Block said. Officials have not identified any of the Marines aboard.
Warren told reporters that the incident will not immediately change the way the way Marines and others use the Osprey.
The MEU is made up of more than 4,500 Marines and sailors. It includes the amphibious assault ship Essex, amphibious dock landing ship Rushmore and dock landing ship Anchorage.
The mishap came as U.S. military officials prepared to host defense leaders from across the Pacific. The gathering was expected to include a demonstration of an amphibious landing.
Staff writer Andrew Tilghman and USA Today contributed to this report.