Army air units in the US have been grounded for training after deadly helicopter crashes in Alaska and Kentucky killed 12 soldiers in the past month.
Operations have been suspended with immediate effect until personnel undergo instruction.
For active-duty units, the training will take place in the coming week, while national guard and reserve troops will have until 31 May to complete the course.
"The move grounds all army aviators, except those participating in critical missions, until they complete the required training," the US Army said in a statement.
On Thursday, two army helicopters collided near Healy, Alaska, killing three soldiers and injuring a fourth.
The aircraft from the 1st Attack Battalion, 25th Aviation Regiment at Fort Wainwright, near Fairbanks, were returning from training at the time of the crash, according to the army.
The unit is part of the 11th Airborne Division, which is nicknamed the Arctic Angels.
Military investigators have travelled to the scene of the crash.
The army on Thursday said two of the soldiers died at the site and the third on the way to a hospital.
The injured soldier is said to be in a stable condition
Referring to the decision to ground flight units for training, army chief of staff James McConville said: "The safety of our aviators is our top priority, and this stand down is an important step to make certain we are doing everything possible to prevent accidents and protect our personnel."
The crash is the second accident involving military helicopters in Alaska this year.
In February, two soldiers were injured when an Apache helicopter rolled after taking off from Talkeetna.
In March, nine soldiers were killed when two US Army Black Hawk medical evacuation helicopters crashed during a routine night-time training exercise about 30 miles (50km) northeast of Fort Campbell, Kentucky.
Speaking at the time, US Army secretary Christine Wormuth called it a "heavy day", with the accident one of the worst for the military in recent years.
The army has said while Thursday's crash and the one in Kentucky remain under investigation, "there is no indication of any pattern between the two mishaps".