The Marine Corps said Friday that all its aircraft units are required to halt flights for 24 hours and have a refresher course on safe operations following two deadly crashes this summer.

Commanders have some flexibility on when aircraft will be grounded, but the training must be completed over the next two weeks across the Marine Corps, said Gen. Robert Neller, the commandant of the service.

"The intent is for flying squadrons to review selected incidents which occurred enterprise-wide and study historical examples of completed investigations in order to bring awareness and best practices to the fleet," the service announced.

Three Marines were killed last week when a tilt-rotor MV-22 Osprey aircraft went into the ocean off the coast of Australia during a training mission. Separately, 15 Marines and a Navy corpsman were killed July 10 when a Marine KC-130 refueling tanker smashed into a field in Mississippi.

The Marine Corps said such pauses are not unusual and that the groundings would be done so as not to affect military commitments.