The U.S. Army has grounded aviation units for training after a series of deadly helicopter crashes.

The suspension of air operation, effective immediately, will be in place until units complete training, Army spokesperson Lt. Col. Terence Kelley told the AP. Training for active duty units is set to take place now until May 5. Army National Guard and Reserve units have until May 31 to complete the training.

“The move grounds all Army aviators, except those participating in critical missions, until they complete the required training,” the Army said in a statement.

A total of 14 soldiers have died and three have been injured in a series of helicopter crashes in recent months. The most recent involved a mid-air collision between two U.S. Army AH-64  helicopters that took place near Healy, Alaska last week. Three soldiers were killed and one was injured.

Two more soldiers were injured in Alaska on Feb. 5 when a military helicopter was injured in a rollover accident in Talkeetna, Alaska.

On March 29, nine soldiers were killed in Kentucky when two Black Hawk medical evacuation helicopters crashed during a routine nighttime training exercise. Alabama native Staff Sgt. Taylor Mitchell, 30, of Mountain Brook, died in that crash. The 101st Airborne confirmed the crash was about 30 miles northwest of Fort Campbell.

And, on Feb. 15, a Black Hawk helicopter from the Tennessee National Guard crashed in Madison County, Alabama during a flight-training mission, killing two crew members.