WASHINGTON - Two US soldiers died in a helicopter crash during training at Fort Campbell, Kentucky, the Army said on Saturday, marking the fourth US military aircraft mishap in less than a week.The crew of the AH-64E Apache helicopter were conducting routine training exercises when the crash occurred on Friday night, according to an Army statement posted to Fort Campbell's Facebook page.The solders, both from the 101st Airborne Division, were not identified. The Army said the incident was under investigation."This is a day of sadness for Fort Campbell and the 101st Airborne," Brigadier General Todd Royar, acting senior commander of the 101st Airborne Division and Fort Campbell, said in the Facebook statement. "Our thoughts and prayers are with the families during this difficult time."The crash is the latest in a series of similar incidents involving US military aircraft that began on Tuesday.A US Marine Corps helicopter crashed during training in Southern California on Tuesday. All four members were believed to be killed.A Marine jet also crashed in Djibouti on Tuesday. The pilot was later reported in stable condition.On Wednesday, a US Air Force pilot with the traveling Thunderbirds exhibition squad died when his F-16 fighter jet crashed in Nevada.