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The US Army has grounded all aircraft except those on "critical missions" after two recent fatal mid-air crashes.

The "aviation stand down" order announced on Friday comes a day after two Apache helicopters collided in Alaska, killing three US Army pilots.

Army Chief of Staff James McConville said the aviators will remain grounded until they complete extra training.

A crash on 20 March involving two Black Hawk helicopters in Kentucky left nine soldiers dead.

"We are deeply saddened by those we have lost," General McConville said in a 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."

He added Army pilots "will focus on safety and training protocols to ensure our pilots and crews have the knowledge, training and awareness to safely complete their assigned mission".

The stand down is effective immediately. It does not affect any other part of the US military besides the Army.

Active-duty troops will be required to complete their training within 24 hours in the first week of May while National Guard and Reserve units have until the end of the month.

As each unit reports having done the training they will be able to return to normal operations.

The recent incidents, which occurred in different parts of the country, are under investigation but officials say "there is no indication of any pattern".

The crash on Thursday occurred near Fort Wainwright, Alaska, as the troops were returning from a training mission, the Army said. In addition to the three that died, one was critically injured.

Three troops were part of the 11th Airborne Division, which is nicknamed the "Arctic Angels", according to the Associated Press.