SYDNEY:Five Australian troops have been killed in two separate incidents in Afghanistan, defence officials said Thursday, in the worst day for Australian casualties since the war started.

In one incident, two Australian special forces soldiers were killed Thursday morning when their helicopter crashed in Helmand province, in one of the worst days for Australian casualties since the Vietnam War.

The news came after the International Security Assistance Force (Isaf) said three soldiers in the Nato-led force in Afghanistan had been killed by a member of the Afghan security forces.

The attack occurred Wednesday evening in southern Uruzgan province where about 1,500 Australian troops are deployed.

“Defence can confirm that Australian Defence Force personnel have been killed in Afghanistan,” the ADF said in a statement.

“Defence is currently in the process of informing the next of kin of the ADF personnel involved. The acting chief of the defence force will make a statement once this process is complete.”

Earlier, Isaf said the latest killings were an “insider attack” that has caused growing dismay among coalition commanders.

“An individual wearing an Afghan National Army uniform turned his weapon against International Security Assistance Force service members in southern Afghanistan yesterday, killing three,” Isaf said in a statement. “The incident is currently under investigation.”

Nato has struggled to counter the so-called “green-on-blue” attacks in which uniformed Afghans turn their weapons against their international allies.

The assaults have spiked this year, with more than 30 attacks claiming the lives of 45 coalition troops, comprising about 14 per cent of the overall death toll in the war for 2012, according to Isaf.

A close ally of the United States, Australia's Afghan deployment began in 2001 before Canberra pulled out, only to redeploy to the arena in 2005.

Before the latest killings, 33 Australian lives had been lost in the war.