The United States Army’s chief of staff has grounded all pilots not involved in critical missions until they complete required training after four helicopters crashed in a matter of weeks with multiple deaths.

The suspension of air operations was effective immediately on Friday, with units grounded until they complete the training, said Lieutenant-Colonel Terence Kelley, a US Army spokesperson. For active-duty units, the training is to take place between May 1 and 5. Army National Guard and Reserve units will have until May 31 to complete the training.

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.

Much is still not publicly known about the crash in the remote interior of the state that is 402 km (250 miles) from Anchorage.

The Army said two of the soldiers died at the crash site and a third on the way to a hospital in Fairbanks. A fourth soldier was injured and taken to a hospital. That soldier was in stable condition on Friday, said John Pennell, a spokesperson for the US Army Alaska.

US General James McConville “ordered an aviation stand down following two deadly helicopter mishaps that killed 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 on Friday.

“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.

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 neighbourhood near Fort Worth, Texas in 2021. The pilots ejected before the plane went down. (Al Jazeera)

•PHOTO:A US Army Apache AH-64D attack helicopter in flight. Two Apaches collided in Alaska this week killing three soldiers and injuring a third [File/AP]