A female sergeant major on board the military aircraft was injured but is expected to recover.
The dead include a major general, a colonel, a major and a lieutanant, the military high command said in a statement.
The Huey UH-1H helicopter had left early Wednesday from Larissa, in the centre of the country, heading north toward Kozani, army spokesman Colonel Nikolaos Fanios earlier told AFP.
State television ERT said the helicopter likely hit power cables.
Multi-purpose helicopters, Hueys are among the oldest aircraft in Greece's arsenal. The model first saw action with the US armed forces during the Vietnam War in the 1960s.
Three Greek soldiers died last year when their Agusta Bell navy helicopter crashed into a small Aegean Sea islet during a pre-dawn exercise.
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