Two AH-64 Apache helicopters collided in Alaska on Thursday, killing three soldiers and injuring a fourth, while two Black Hawks crashed in Kentucky late last month, leaving nine dead.

General James McConville "ordered an aviation stand down following two deadly helicopter mishaps that claimed the lives of 12 soldiers. The move grounds all Army aviators, except those participating in critical missions, until they complete the required training," the Army said in a statement.

"During the stand down, the Army will review the risk approval/risk management process, aviation maintenance training program, aircrew training standardization and management, and supervisory responsibility," the statement said.

Active-duty units must complete the 24-hour stand down between May 1 and 5, and National Guard and Reserve units by May 31, it added.

There have been multiple other crashes of US military aircraft in recent years, including one involving a Black Hawk that killed two Tennessee National Guardsmen during a training flight in Alabama in February.

Four US Marines were killed during NATO exercises in Norway last year when their V-22B Osprey aircraft went down, possibly after hitting a mountain, investigators said.

And two US Navy pilots were rescued after their T-45C Goshawk jet crashed during a training exercise in a residential neighborhood near Fort Worth, Texas in 2021. The pilots ejected before the plane went down.