The government of Djibouti ordered the grounding of military flights following the incidents, said Commander Bill Urban, a spokesman for the US Navy's Fifth Fleet.

As a consequence an amphibious training exercise known as Alligator Dagger was canceled.

"A safety stand-down has been initiated for all exercise participants," Urban said.

The US military has been rocked by a string of aviation accidents in recent days, including an F-16 crash near Las Vegas on Wednesday that killed the pilot.

On Tuesday, four crew members died when a Marine Corps helicopter crashed while on a routine training mission in Southern California.

And in Djibouti, two incidents on Tuesday triggered the grounding of US military flights.

The first saw an AV-8B Harrier jet crash at Djibouti International Airport. The pilot, who ejected, survived the crash.

Then separately, a Marine CH-53 Super Stallion helicopter suffered "structural damage" during a landing at a landing zone at Djibouti's Arta Beach, Urban said. The crew in that case was not injured.

The US base in Djibouti is a key a staging area for the US in its anti-jihadist campaigns in Yemen and Somalia.

Pentagon spokesman Marine Lieutenant General Kenneth McKenzie said there was no apparent linkage between any of the accidents.

"We regret each one. We'll look at them carefully. I'm certainly not prepared to say that it's a wave of mishaps or some form of crisis," McKenzie said.