KABUL, Afghanistan — Five Australian soldiers were killed in southern Afghanistan within hours of one another on Wednesday and Thursday, three of them at the hands of a turncoat Afghan soldier, making it the deadliest period in a decade of fighting here for one of the United States’ staunchest allies.

Two soldiers died when their UH-1 Huey helicopter crashed in the Baghran district of Helmand Province on Thursday morning, NATO officials said. They said they did not know what caused the crash, which left other soldiers injured as well.

Three other soldiers were killed on Wednesday night when an Afghan soldier turned his gun on them in Oruzgan Province, the latest in a rash of insider attacks on coalition forces that have left 45 dead this year at the hands of the Afghan security forces or other Afghans working with them.

The attack happened at a fuel depot when a member of the Afghan National Army shot the Australians and then fled the base, coalition officials said. The international force command said that the motive was unclear and that it was investigating.