The US army on Friday grounded all aircrafts except those on critical missions, provisionally for about a week, after four fatal helicopter crashes within the last month.

General James McConville "ordered an aviation stand down...until they complete the required training", the army said in a statement.

"The safety of our aviators is our top priority," said McConville.

For active units, the training is to be completed between May 1 and 5, while National Guard and Reserve units will have until May 31, the army said.

The training will "review the risk approval/risk management process, aviation maintenance training program, aircrew training standardization and management, and supervisory responsibility," the statement said.

The US Army only operates combat helicopters, with US combat planes part of either the Air Force or the Navy.

Multiple crashes in recent months

In the last month, a total of 12 soldiers have died in helicopter crashes.

The latest crash took place in Alaska on Thursday when two AH-64 Apache helicopters collided, killing three soldiers and injuring a fourth.

And in early April, nine soldiers died in Kentucky when two US Black Hawks crashed during what was described as routine night training.

The US army said that while Thursday's crash in Alaska and the one in Kentucky are currently under investigation, "there is no indication of any pattern between the two mishaps."

Prior to that, 2 soldiers were killed in February when a Black Hawk crashed in Alabama.

Four US Marines from the Navy were also killed during a NATO exercise in Norway last year after their V-22 Osprey helicopter went down.