Our thoughts are with the injured personnel at this time and the incident will be thoroughly investigated

Army spokesman

Two people being treated in the Hospital Victoria Hospital in Belfast are described as being seriously ill.

The helicopter, based at RAF Aldergrove in County Antrim, had been on a routine flight between security force bases in south Armagh.

The Army said the cause of the accident was unknown, but that terrorist involvement had been ruled out.

A board of inquiry has begun a full investigation into the crash.

An Army spokesman said: "Our thoughts are with the injured personnel at this time and the incident will be thoroughly investigated.

"I can, however, rule out terrorist involvement.''

Local Sinn Fein councillor Packie McDonald, who witnessed the incident, said the helicopter appeared to go into a spin as it tried to land at a military observation post.

"The pilot tried to control it for a while but shortly after it just crashed on the side of the mountain about 150 yards from the look-out post.

"The tail has broken off and the propeller is lying on the ground.

"The main part of the helicopter is lying on its side."