BBCA Royal Navy helicopter has crashed into a field in an incident police have described as "ongoing".
Devon and Cornwall Police said emergency services were at the scene in Sourton Down near Okehampton in Devon.
Military helicopters often train in the area around north Dartmoor, and the Ministry of Defence confirmed it belonged to the Royal Navy.
Roads in the area were closed at about 05:00 BST while the incident is investigated.
National Highways said the closure of the A386, between the A30 at Sourton Down and the A3079 Fowley Cross in Okehampton, is expected to be in place until 12:15.
Staff at Sourton Cross Service Station, which is located near the scene, said the services were currently inaccessible.
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