3 Australian troops killed in 'green-on-blue' attack; 2 in Afghan chopper crash

KABUL, Afghanistan (CNN) -- In separate incidents within the span of a day, five Australian troops were killed in Afghanistan: three in an apparent "green-on-blue" attack and two in a helicopter crash, officials said Thursday.

"This is our single worst day in Afghanistan," Australian Prime Minister Julia Gillard said.

In the first incident, someone wearing an Afghan National Army uniform shot and killed three soldiers inside their base in southern Uruzgan province late Wednesday

Two others were wounded.

Pentagon: Afghans killing U.S. troops

The soldiers were in a group "relaxing and talking" at the end of the day when the incident occurred, Air Marshal Mark Binskin, the acting chief of the Australian Defence Force, told reporters

U.S. facing growing 'green-on-blue' challenge

Other soldiers returned fire, but the shooter escaped by scaling a fence, Binskin said.

The deaths bring to 14 the number of NATO troops who have been killed in August alone in such "green-on-blue" attacks.

The other two Australian soldiers were killed when the International Security Assistance Force helicopter they were riding in crashed while trying to land in Helmand province Thursday morning.

"It appears that the helicopter has landed and rolled over, and that's all I know at the moment," Binskin said, adding there was no indication that enemy action was to blame.

Until now, Australia had suffered only one military fatality in Afghanistan in 2012.

"I think morale at the moment has taken a hit over this understandably," Binskin said.

Earlier this year, Australia said it would pull out its more than 1,500 troops from Afghanistan ahead of schedule.

There has been a recent surge in "green-on-blue" attacks, or insider attacks in the country.

The term refers to a color coding system used by the military, in which blue refers to the friendly force, in this case ISAF; and green refers to an allied friendly forces, in this case Afghan National Security Forces.

Concerned about the uptick, U.S. Gen. John Allen, ISAF's chief commander, ordered all allied forces at NATO headquarters in Kabul and all bases across Afghanistan earlier this month to carry loaded weapons around the clock.

General: Quarter of Afghan insider attacks by Taliban

Earlier this month, President Barack Obama said he is "deeply concerned" about the growing number of deadly attacks on U.S. forces by Afghan security forces.

Obama said he would talk to Afghan President Hamid Karzai to try and stop these attacks.

A paper weapon to defend against 'green on blue' attacks

CNN's Ab Qadir Sediqi in Kabul contributed to this report.