An AH-1Z Viper helicopter similar to the one that crashed (Picture: Flickr/Gerry Metzler).
Two US Marine pilots have died during a routine training exercise in the skies of Arizona at around 04:45 GMT on Sunday.
Major Matthew M. Wiegand, 34, of Ambler, Pennsylvania, and Captain Travis W. Bannon, 30, of Nashville, Tennessee, lost their lives when their AH-1Z Viper aircraft came down near the Marine Corps Air Station at Yuma Proving Ground in Arizona.
They were participating during a course hosted by Marine Aviation Weapons and Tactics Squadron One (MAWTS-1).
The Marines tweeted to say: "Our condolences go out to the families and friends of our two pilots who we lost last night in Yuma, Az.
"We ask that you please respect them during this time."
Tributes also came from a number of other sources.
"It is a somber day for the entire Marine Air Ground Task Force Training Command (MAGTFTC) as we mourn this tremendous loss," said Commanding General, MAGTFTC, Brigadier General Roger B. Turner Jr. He continued:
"Our thoughts and prayers remain with their families and loved ones during this extremely difficult time."
The cause of the crash is currently under investigation.