▶ Watch Video: Army orders aviation standdown after deadly helicopter crash

The U.S. Army on Friday grounded all aircraft — except those in “critical missions” — following a mid-air collision Thursday involving two helicopters in Alaska which left three soldiers dead and a fourth wounded. It marked the second such mid-air collision in the last month.

The Army said in a news release provided to CBS News that Chief of Staff James McConville ordered the “aviation stand down” after the two recent fatal accidents which claimed the lives of 12 soldiers, including a mid-air collision involving two Black Hawk helicopters near Fort Campbell, Kentucky, March 29 which left nine soldiers dead.

All aviators, except those in critical missions, are grounded until they “complete the training,” the Army 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,” said McConville in a statement.

Thursday’s mid-air collision involved two AH-64 Apache helicopters that were returning from a training mission near Fort Wainwright, Alaska, the Army disclosed.

And in February, two members of the Tennessee National Guard were killed when a Black Hawk helicopter crashed during a training mission near Huntsville, Alabama.

“We are deeply saddened by those we have lost,” McConville said in his statement. “It is their loss that makes it all the more important we review our safety procedures and training protocols, and ensure we are training and operating at the highest levels of safety and proficiency.”

Active-duty units will be required to complete the training between May 1 and May 5, while units in the National Guard and Reserve will have until May 31, the Army said.