Bogota: The military plane crash that killed 69 service members last month in Colombia was caused by the aircraft hitting trees, the air force said Thursday in preliminary findings from an investigation.

On March 23, the C-130 Hercules plane, manufactured in 1983, crashed one kilometre from the runway it had taken off from in the Amazonian town of Puerto Leguizamo, near the border with Peru.

Colonel Luis Fernando Giraldo, safety director of the Colombian air force, told reporters the plane struck three trees just four seconds after takeoff.

The investigation revealed "vegetation material" inside two of the engines, he said.

That "would have affected the aerodynamic performance of the left wing and the control of the aircraft," Giraldo said.

The aircraft was carrying more than 120 people, as well as ammunition, which exploded and injured nearby residents who rushed to rescue injured survivors.

Giraldo said it is not yet possible to determine whether the crash was due to human or mechanical error, but ruled out excess weight or adverse weather conditions as factors.