KABUL, AFGHANISTAN: Five Australian soldiers were killed in southern Afghanistan within hours of one another 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 staunchest U.S. allies.

Two soldiers died when their UH-1 Huey helicopter crashed in the Baghran district of Helmand province 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 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 the motive was unclear and that it was investigating.

With 1,550 troops in Afghanistan — most of them in Oruzgan — Australia has the largest non-NATO military presence in the U.S.-led coalition in Afghanistan. The only other Australian military fatality in Afghanistan this year was in July.

Last year, 11 Australian service members were killed in Afghanistan, according to Icasualties.org.

The five new deaths stunned Australia. Prime Minister Julia Gillard called it “the most awful news” for the country.

“This is a very big toll,” Gillard said during a visit to the Cook Islands, where she said she would cut her trip short to return to Canberra, the Australian capital. “This is our single worst day in Afghanistan.”

She said insider attacks like the one that killed the three Australians were “corrosive of trust” and difficult to deal with.

This year, Gillard announced that Australia would withdraw its troops by the end of 2013 — one year ahead of schedule — citing what she said were security improvements in Afghanistan, while also acknowledging the unpopularity of the war.

On Thursday, she vowed that the five deaths would not alter that plan.