Afghan security personnel in Kabul Oct. 6, 2015.(Photo: Noorullah Shirzada/AFP/Getty Images)

Two U.S. servicemembers were among five killed Sunday when a British helicopter crashed in Afghanistan's capital, Kabul, the third mass-casualty event involving transport aircraft in the war-torn country in three months, coalition forces in Afghanistan said Monday.

The other dead were two British servicemembers and a French civilian contractor in what the NATO-led military mission described as a "non-hostile incident." The crash involving a British Puma Mk 2 helicopter happened at 4:15 p.m. local time at Camp Resolute Support in Kabul, and an investigation is underway.

"We're deeply saddened by the loss of our teammates," said U.S. Brig. Gen. Wilson Shoffner, military communications chief in Afghanistan. "Our heartfelt condolences go out to the families and friends of those affected in this tragic incident and we pray for the full recovery of the injured."

An Afghan security guard who witnessed Sunday's crash told the Associated Press the helicopter struck a monitoring balloon as it was landing. "It seemed like the cable was turned around the helicopter rotor," the security guard, Najibullah, said. Like many in Afghanistan, he goes by one name. He said the helicopter went down and then black smoke rose from the area.

A second helicopter circled the area three or four times and then landed at a nearby airport, he said.

U..S. Army Col. Brian Tribus, a spokesman in Afghanistan for U.S. and NATO forces, confirmed the accident involved a NATO aircraft and an observation balloon, according to the Military Times. The monitoring balloon was severed from its mooring in the incident, he said.

On Oct. 1, a C-130 transport plane crashed near Jalalabad, killing six U.S. servicemembers and five civilian contractors, as well as several Afghan civilians on the ground. At the time, President Obama said the deaths served as a reminder "of the sacrifice brave Americans and our Afghan partners make each and every day in the name of freedom and security."

A helicopter crash in a remote region in southern Afghanistan in August killed 17 Afghan soldiers. That crash was initially blamed on technical problems and was among the deadliest events for the Afghan military since U.S. and NATO forces concluded their combat missions and shifted to a support and training role last year.

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