3 NATO troops killed in Afghanistan in apparent 'green-on-blue' attack

KABUL, Afghanistan (CNN) -- Someone wearing an Afghan National Army uniform fired on NATO soldiers and killed three of them, NATO's International Security Assistance Force said Wednesday.

The shooting happened Tuesday and is under investigation, it said.

Two ISAF troops were killed in a similar attack Monday by another Afghan soldier. Fellow ISAF soldiers then shot and killed the gunman.

There has been a recent surge in such "green-on-blue" attacks, or insider attacks, by Afghan security forces against U.S. and other allied troops in Afghanistan.

Concerned about the uptick in the attacks, 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.